Mars Core

Mars Core is the seventh level of Doom Eternal. Beginning at the orbital defense base on the moon of Phobos, the Doom Slayer battles an ongoing invasion while intending to reach the core of Mars, where an ancient Night Sentinel slipgate resides. Through the use of an orbital weapon system known as the BFG-10000, the planet Mars itself suffers massive damage, directly creating a path to its core.

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 Doom 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 entering outside.
 * The exclusion of a One-Up outside.
 * The ballista's arbalest weapon mod has the interface of what would look like the destroyer blade's zoom reticle during the reveal.
 * 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, in which it is undergoing refinement phase or development at that time.
 * 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.
 * It is possible to skip the chaingun through the use of glitches and exploits.
 * The entrance to the BFG-10000 is accessible in the reveal, which reveals an arch-vile at the other side of the door with the Doom Slayer pulling out and swinging his Crucible blade as the reveal ended in a cinematic fashion. In the final version however, the entrance is inaccessible, as the maintenance hatch is taken instead. Both the arch-vile and the Crucible blade are also introduced later.
 * The exclusion of the ice bomb throughout all playthroughs, likely under development. The aforementioned flame belch takes its place when switching equipment types.
 * Ammo count is increased, on par to Doom 2016 as opposed to low ammo capacity to facilitate the use of the chainsaw.
 * The Heads-Up Display is akin to Doom 2016, as opposed to its final version.
 * In addition, weapons with certain weapon mods will zoom the HUD alongside with the screen. It briefly disappears on some sequences (such as starting to climb a wall).

Another portion of the game's pre-release taking place in QuakeCon 2019, during Mars Core is similar to the final version, with more sequences shown.