I was thinkin' of inserting some environmental crap in my project where the aliens on the underground were once like us that lost their planet.
Explaining it further, there's an underground world accessible through a historical site. Actually, the underground world was a synthetic Earth (think of Tokyo Underground, and/or the Neo Ark in Parasite Eve 2). The DESU! (superheroes) get in to investigate. At first few floors, they went to sewers, then to a cavern, and then some hints of alien civilization and finally, the underground world itself.
Would that breach the Underground theme?