Monolith

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

Unveiling these will spoil the experience of finding them yourself.

Description of the location(s)

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.

Screenshot

Inscriptions

Mars

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

Moon

zer0's Legion: SOLVED: the BIGGEST mystery in SE (2024)

Stereolith in the game "Space Engineers" has not been solved

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.