The Animorphs Media
The Yeerk pool is a cesspool of liquid the color of molten lead where the Yeerks must return to every three days to absorb Kandrona Rays. There are two steel piers leading out over the pool, where Hork-Bajir controllers drag temporarily uninfested hosts from cages into the pool to be reinfested when the Yeerks are done feeding. The Earth Yeerk pool is located in a giant, ever expanding underground complex which spans beneath the entire hometown of the Animorphs. It's frequently attacked, damaged and towards the end of the books, outright destroyed. The Yeerk pool, along with the Kandrona, is similar to a church, or other house of worship to the Yeerks.