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 conventional travel would take too long, 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."

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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 megasphere.
 * 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 a 1-Up 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.