Moist Dirt is an in-game feature of antwar.io, and it can be found underground. There are markings on the surface that indicate the presence of moist dirt underground. It's the appropriate spot to store items that are part of ant development, such as eggs and larva, as they will otherwise spoil.
After 500 "uses" the tile will become normal dirt. 1 use is counted as a single brood being on a tile at the end of an 8 second period. Placing multiple brood on a tile will speed up this process.