Mars Core

Mars Core is the seventh level of Doom Eternal. After learning from Samuel Hayden that Deag Grav, the final Hell priest, is on Sentinel Prime, the Doom Slayer must proceed to Mars to locate a trans-dimensional gateway to Sentinel Prime, hidden in the planet's core within the lost Night Sentinel city of Hebeth. As VEGA confirms there are no pathways leading there, the Slayer travels to Phobos to gain access to its BFG-10000 orbital defense system and blast a hole in Mars directly to the core.

The BFG-9000 weapon is found in this level. The Maykr drone enemy is also introduced in this level via the Slayer Gate.

"With the help of Samuel Hayden, the final Hell priest has been located on Sentinel Prime - a city from your past. The only known path to this now accursed place is through the dimensional gateway hidden in the core of Mars, wherein the lost city of Hebeth resides. The journey there will take time you can't afford to waste - a dilemma which calls for a more...direct approach."

- Loading description

Objectives

 * Shoot a hole in Mars
 * Get to the BFG-10000


 * Evacuate the Phobos base
 * Reach the portal
 * Use the ion catapult
 * Get to the escape pod

The BFG-10000
Once the elevator reaches its destination, kill the prowler and mecha zombies in front of you then enter the passage they came from to dispose of some imps. Look up when you reach the end of the passage to see a vent opening and climb into it, then punch through the cracked grate on your right to enter a tunnel with more imps as well as zombies, kill a pinky that charges at you from the other end of the tunnel then punch through another vent to land in a room with more zombies as well as a blue teleporter. Step on the teleporter to enter a room containing a baron of Hell - try to stay out of melee range as much as possible, either by staying on the move or by using ice bombs and faltering weapons to hold it in place. Head up either set of steps and go through the green door at the top to find a runestone, then turn left and go through the circular airlock to get outside.

There is an arena ahead with various heavy demons including an arachnotron, a cyber-mancubus and some carcasses; once they are defeated, continue moving forward up some steps and kill several blaster soldiers that come through a door, then go up the slope to a second arena where you will find a pain elemental that throws lost souls at you; once the elemental is killed it will be replaced with a dread knight and then a Doom hunter, so use caution. Once the arena is clear, drop down the hatch that rises from a square platform then look down to see a circular opening in the middle of a fan that you can use to get inside the BFG core. Punch the cracked fuse in front of you to lower the energy grids then walk straight through a small opening in the wall behind it to enter a room full of pistons; kill the zombies and gargoyles here, use the automap station then climb on top of the pistons to reach a vent leading out of the room. Exit the vent and turn right to follow a passage with flame jets, kill a mancubus coming the other way (along with a prowler that tries to surprise you from behind), then use the ladder at the end of the passage to reach the BFG-10000.

To the core
After the cutscene, go through the door in front of you and use your new BFG-9000 to kill all the zombies and cacodemons ahead, then step into the open blue teleporter to go down to the destroyed Mars facility. Follow the platforms down until you reach the facility entrance then head straight forward to a door, through which you will see a large ion catapult; do not look at it for too long, as there is a combat arena below you with a buff totem - drop into the arena, turn left and head towards a launcher then look right to find the totem; once the totem is destroyed you can turn your attention to the other enemies, which start with a Hell knight and carcass and end with a mancubus and two whiplashes. After clearing the area, go down the hatch that opens under the center platform then head through a doorway and go straight forward until you get to an airlock with an elevator behind it. Ride the elevator up to a room containing the ion catapult's control panel and interact with it, go through the previously-locked door to an airlock with another runestone behind it (this should be the final rune you acquire), then continue on to the catapult and use it to fire yourself to a building on a nearby meteor.

Kill the soldiers and mecha zombies ahead then go up the steps and turn left to enter a room with several tentacle holes; tread carefully as most of them contain tentacles. Look for a box under an open vent and climb up it until you reach a Night Sentinel spirit holding a Praetor suit point, turn left and jump across to a launcher that will fire you up to an overhead ledge, then drop down the other side to land on a rock platform. The path ahead leads past a number of small generators that produce harmful electric fields, so avoid walking on them as you continue down to a platform with a couple of imps on it. Go down the steps here to find a climbable wall and go down it as far as you can, then turn around to leap to a launcher that will blast you up to a small rock platform; from here you need to turn round and time a leap past some spinning debris to reach the tunnel behind it. Go through the tunnel and kill two cacodemons that fly towards you, then jump down and dash towards a large building where some mecha zombies are shooting at you. Two barons will spawn as soon as you land, but there is a unit of BFG ammo on the upper floor that you can use if needed.

Once the barons and other enemies are dead, go through the large door on the lower floor and walk past a pair of moving lasers - a pain elemental will crash through the ceiling and it will be joined by several gargoyles, so kill them all to open the next door and go outside to dispose of two cacodemons. Make your way down to a large platform with a rotating tower and kill all the enemies there (including another baron), then continue onwards and downwards to a large platform that has a ledge with several zombies running round it. Head round this ledge to see a couple of climbable walls (the right-hand one is faster, but the one between the columns can lead you to an extra life) and go up to a platform with some more demons, then make your way to the top floor and use a launcher to fire yourself over to a large building. You will see a wall explode as you land on the walkway - jump across to this wall to find an airlock leading inside the building, then go through and clear the next room of tentacles before dealing with a revenant and Hell knight that enter the area.

Head through the tentacle area until you get to a room full of barrels and boxes, then drop down the hatch here and head forward to find the escape pods - the green-lit pod can be used to get yourself down to Mars. After the cutscene, follow the path down into a stone ruin and punch through a green-cracked wall to enter the final arena - you will have to contend with various Hell nobles followed by an arachnotron and then a Doom hunter. Once all significant opposition has been destroyed the gate at the back of the arena will open - go through it and use the BFG ammo provided to blast any remaining opposition, then cross the platforms until you reach a launcher leading up to the slipgate for Sentinel Prime.

Slayer Gate
In the final building of the meteor section are several rooms with tentacle holes. One of these rooms has some steps leading up to a computer terminal with a distinctive orange button; press it and look through the window ahead to see a wall support move, then go round to that area to find that a block has risen letting you get up to two cracked vents. One of the vents has the "QUAKE CHAMPIONS - Goroth" album behind it, while the other leads to the Slayer Key. As soon as you get the key, drop down and turn left, go forward round a corner then turn right to find the Slayer Gate.

This encounter initially pits you against a marauder along with prowlers and various fodder demons, including a Maykr drone - drones can be quickly killed with headshots from the heavy cannon's Precision Bolt or the combat shotgun's Sticky Bomb, but make the marauder your priority due to its strength. Once the marauder is killed, it will be replaced with four Hell knights followed by two barons.

Challenges

 * Big Ba-da Boom - Kill 40 demons with a very big gun
 * Disarmament - Destroy three arm cannon weak points on mancubi
 * Lock and Key - Find the Slayer Key

Secret encounters

 * 1) A gore nest can be found in the same area as the second Praetor suit point. You will have 25 seconds to kill a revenant, a spectre and some fodder demons.
 * 2) After jumping over the room full of tentacle holes and landing on a rock ledge on the other side, head round the building in front of you to go inside it and find a cracked grate hiding a blue teleporter - step on the teleporter to reach a gore nest. You will have 30 seconds to kill several imps along with two whiplashes, a cacodemon and a prowler.

Special items

 * Six extra lives
 * A 1UP can be found above a bottomless drop in the first outdoor arena.
 * As soon as you enter the Mars facility building, turn right and follow the passage to a 1UP.
 * From the third Praetor suit point you will use a launcher to jump to an overhead ledge. There is a monkey bar above this ledge, and if you swing off it you can dash forward to a cracked wall hiding a 1UP. (If you miss it, you will drop into this room after leaving the second secret encounter.)
 * In the middle of the meteor section is a tower with two rotating fins, which has the 1UP on it. Just before reaching this tower you can use a platform with a launcher to fire yourself across to a second platform with an electric generator, then make a carefully-timed jump past the fins to reach the pickup.
 * From the platform with the third BFG ammo pickup, drop down to a ledge with zombies and follow it round until you see a climbable wall between two metal columns. Jump to that wall, turn left to reach a second wall then turn right to reach a 1UP beside a third wall.
 * The last 1UP is above the final arena before the slipgate, reachable using a floating monkey bar
 * Four Argent cells
 * In the outdoor area immediately after acquiring the BFG.
 * On the upper level of the arena where you face two barons.
 * Just after the tower with the fourth extra life are some floating rock columns that have climbable walls on them. Head along the walls until you see a platform on your left holding the ammo pickup, which is surrounded by electric generators.
 * Just before the slipgate.

Collectibles

 * One Sentinel crystal
 * As soon as you reach the large building at the end of the meteor section, go through the doorway closest to you to find the crystal.
 * Five Praetor suit points
 * In the BFG core, a Night Sentinel spirit can be found on the upper level of the piston room, in the opposite corner from the exit vent.
 * After clearing the arena that had the first buff totem, drop down the hatch in the floor then look for a cracked wall in one corner. Punch through this wall and drop down to a platform below you, then go up some steps to a Night Sentinel.
 * A Night Sentinel is encountered after climbing to the platform above the room full of tentacle holes.
 * From the third BFG ammo pickup, look back the way you came to see a rock column with a large climbable wall on it. Go up this wall to get on the column (watching out for an electric generator), then jump across to a platform with a medikit before leaping to another platform with a Night Sentinel.
 * Just before reaching the final arena before the slipgate you have to punch through a cracked wall. Ignore that wall for now and turn left to find a Night Sentinel in an alcove.
 * Two Codex pages
 * "BFG-10000" - as soon as you exit the vent leading out of the piston room in the BFG core, turn left to find this page.
 * "The Lost City of Hebeth" - on the path leading to the final arena before the slipgate.
 * Three Sentinel batteries
 * After dropping down the hatch leading to the BFG core, turn round to see a battery in the distance which you can reach by jumping over a couple of platforms.
 * After launching yourself to the large building at the end of the meteor section, turn round to see two round platforms leading back to the area you just came from. Jump across both platforms to reach a T-shaped walkway with a battery.
 * After using the escape pod, drop down a ledge then turn right to find a battery.
 * Two runes
 * A runestone can be found behind the exit from the room where you encounter the first baron.
 * A runestone is placed on the bridge leading to the ion catapult.
 * One Empyrean Key
 * Three toys
 * "Prowler" - when you reach the room with the ion catapult's control panel, go through the doorway opposite the panel to find a question mark.
 * "Carcass" - from the second secret encounter, launch yourself up to the floor above to find a question mark guarded by a zombie cultist.
 * "Pinky" - after dropping down the hole leading to the escape pod room, turn left to see a cracked grate and punch through it, then use a launcher to propel yourself up to a locked room with zombies guarding a question mark.
 * One cheat codes
 * "Onslaught" - as soon as you enter the meteor section of the level, go down the gap in the floor next to the staircase then head forward to a launcher next to a tentacle hole. Use the launcher to fire yourself to the floor above and turn round to find a question mark.
 * Two albums
 * "DOOM 64 - Intro" - From the third Praetor suit point you will use a launcher to jump to an overhead ledge. Turn round when you land on this ledge to see a question mark that can be reached using the monkey bar in front of you.
 * "QUAKE CHAMPIONS - Goroth" - Follow the directions above to get the Slayer Key. One of the two cracked vents you can jump to has a question mark behind it.

Master Level
A Master Level version of this mission was released with Update 6.66 on, which makes a number of changes to significantly increase the level's difficulty:
 * The number and variety of monsters is greatly increased, including the whiplash spectre from The Ancient Gods, Part One. Monsters that normally appear later in the game are included, including tyrants and arch-viles.
 * Environmental hazards such as laser grids, heavy fog and purple goo are added throughout the level.
 * The Slayer Gate is retained but cannot be accessed. The Slayer Key is replaced with a soulsphere.
 * Ten challenges are introduced which require the player to complete the level at different difficulties, including Ultra-Nightmare. The "Classic Mode" introduced with the Super Gore Nest is retained here, where the player starts with an unmodified combat shotgun (and the plasma rifle collected in the opening cutscene) and must collect more weapons and modifications as they progress. Completing all ten challenges unlocks a red chrome skin for the plasma rifle. Each individual challenge also has its own cosmetic item unlock.

Pre-release
A shortened version of the first half of Mars Core was the second level of Doom Eternal to be displayed to the public in the BE3 reveal on.

The noticeable differences from the reveal to the final product includes the following:
 * The player takes control of the Slayer before walking inside the elevator, as opposed to starting inside the elevator in the final version.
 * The PA system inside the Phobos outpost has a different voice actor.
 * The exclusion of the green cracks before exiting out of a vent during the reveal.
 * The exclusion of the UAC Spokesperson hologram before exiting outside.
 * The exclusion of an extra life outside.
 * The exclusion of Samuel Hayden and VEGA throughout the level, presumably to avoid spoilers.
 * The exclusion of the Doom hunter and dread knights near the BFG-10000.
 * The exclusion of the chaingun throughout the level, likely being developed at that time. The final version introduces the chaingun as a mandatory pickup in Super Gore Nest to advance the level, although the part of the level itself was not seen at that time, further supporting the theory.
 * The main entrance to the BFG-10000 is accessible in the reveal, which contains an arch-vile. The Slayer pulls out and swings his Crucible blade in a cinematic, after which the reveal ended. In the final version, however, the main entrance is inaccessible and a maintenance hatch underneath is taken instead. Both the arch-vile and the crucible blade are introduced later, on Taras Nabad.

Trivia

 * Development resources and developer-only portions of the level refer to the final Martian core area of the level as "Hell," and indeed, various infernal influences seem to be present in that location in the form of torrents of lava and other small objects which do not obey normal gravity, and a whispering voice that can be heard on the wind when the Slayer first lands his escape pod. This seems to indicate that the nature of the curse on the area is demonic in origin and has lingered on since the fall of the city.