The general concept so far is follows:
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The underground of the station will be an insland, or series of islands, surrounded by lava. A land bridge leads out to lavaland. The rest of the island contains maintenance for the station above, and maybe a few of the less essential rooms, like the library or the bar. An alien excavation sight turned underground bar like Ausops mapped would be interesting I think
The top of the station has walls on the West/North/South, with a gate on the West and South. The wall is surrounded by chasm, which will drop you in the lava lake surrounding the underground island. To the East, at the very edge, are docklike structures leading out into the ocean with heat exchanges wrapped around them to support the supermatter. A bridge leads out to an island with the AI on it.
Ladders and elevators (shuttles) will be spaced throughout the station to reach the underground. In the center, there will be the gate room (or the shuttle landing pad if the gate concept is postponed/dies).
Progress on specific rooms:
East (main) Gate concept