Carcosa

Description
Carcosa was thought to be a naturally occurring hothouse planet until an asari expedition discovered palatial ruins on its barren surface. Likely the seat of an ancient city, the crumbling edifice dates back more than 2.7 million years. The only intact chamber appears to be a throne room, overlooking a large depression believed to have once been an enormous lake.

There is evidence that the people of Carcosa polluted their planet until it became uninhabitable, triggering a runaway greenhouse effect and spoiling the planet's fresh water supply. Research into the planet's long history will have to wait—asari scientists evacuated when word of the Reaper attack reached the system, leaving behind data drones to explain their findings to anyone who survives.