The Adventure Seed
Aquarius Station is the most ambitious space colony to date. A large nickle-iron asteroid near the orbit of Jupiter has been reworked into a hollow sphere with an interior lake running around it's middle. Construction crews are now building access ports and refurbishing the inside for human habitation.
Except that some members of the crews have gone missing. Witnesses claim that giant tentacles grabbed them and drew them into the water. But that, of course, is impossible; no terrestrial creatures have been introduced into Aquarius. Searches for either the missing workers or the alleged cephalopod have found nothing. Unless you believe that egghead scientist who keeps rambling on about creatures existing in higher levels of dimensions than our own.
You are a group of troubleshooters sent to investigate the mystery on Aquarius. Or perhaps you are members of a construction crew facing the horror first-hand; or members of a supply ship stranded on Aquarius because the panicked administrator has declared an embargo. Any way, you're going to have to grapple with the Tentacles of Space!!!