- "The Yeerk pool. That vast cavern the size of three Astrodomes. A Yeerk complex underground. Beneath the foundations of half our city. Storage and control center buildings. Docked spacecraft. Diverse alien species moving rapidly about their business, united in a common goal: the conquest of humanity. And there, at the center, was the pool itself. Sludgy, leaden liquid teaming and churning with Yeerk slugs. In their natural state. Vulnerable there, and there only."
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. The Yeerk pool, along with the Kandrona, is similar to a church, or other house of worship to the Yeerks.
While involuntary hosts stay in cages while their Yeerks are feeding, voluntary hosts get to stand up and walk away, then get to stay in a nice room with comfortable chairs and TVs. There are two steel piers over the pool - one where the Yeerk leaves the host, and one where the host is reinfested after the Yeerk is done feeding. Around the edge of the cavern, there are buildings which house offices, storage rooms, and a lunchroom with a kitchen where humans can eat.
Animorphs' Hometown Yeerk Pool
- "The Yeerk pool is in a huge underground cavern. It's practically a small city down there. It's under our school, but it's so big that it also runs beneath the fire station, a couple of gas stations, and most of the mall."
"Yeerk pools are generally large and elaborate. They are an important part of Yeerk life. The centers of their lives, really. The pools are, for the Yeerks, what forests and meadows are to Andalites."
- ―Marco and Ax
The Yeerk Pool is a place the Animorphs return many times for missions, because it is the Yeerks' greatest weakness, and the place all Yeerks must go in order to stay alive. Tobias and Ax, because they don't have to go to school, often spend their days tracking potential Controllers to see if they go to the Yeerk Pool, either through a known entrance or a new entrance that the Animorphs can potentially make use of in future missions.