From BZPB Wiki

Vassilan is the home planet of the Vassila. It began millions of years ago as a small water world inhabited only by various forms of fish but as time moved on land masses appeared on the surface of the water, prompting the fish to evolve to become humanoid amphibians.

Most of the planet is still covered in water, and it rains a lot, meaning that the landmasses are damp and mossy. Huge testing and storage facilities appear intermittently across its landscape.