Monolith

There's a Steam achievement called "Monolith". The relevant monolith can only be found in the following Custom Games: Earth Planet, Mars Planet, Alien Planet, Moon Base. There is no monolith in the Star System custom game.

The introduction of this mysterious artefact is probably alluding to the movie Space Odyssey 2001. The First Jump Scenario tells the artefact’s background story as part of the Space Engineers lore.

Trivia

The second monolith is jokingly called stereolith, alluding to the fact that mono audio has one channel and stereo has two (or more) channels. While mono- indeed means one, stereo- does not mean two, but it means spatial (3D audio).

Spoilers

Spoiler: description of the location(s) (Survival/General)
There is a monolith on the surface of Mars and another one on the surface of the Moon. They are small and not easy to spot.

Spoiler: Creative Mode/Admin Cheat
You can cheat by finding the monolith (sometimes listed as "platform") in a custom game's Entity List on the Admin Screen, and teleporting yourself there.

Spoiler: Screenshot

Spoiler: Mars Inscription
The Mars monolith's inscription says, A great race has lived on this planet, now they live in th(e) stars.

Spoiler: Moon inscription
Stereolith in the game "Space Engineers" has not been solved

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