The Underground Illustration

The 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[src]

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.

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. The Animorphs will often use these rooms to hide or demorph.

The Yeerk Pool is a place the Animorphs return many times for missions, because it is the Yeerk's 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. 



Yeerks swimming in the Yeerk Pool

In chapter 5 of The Invasion, Visser Three claims "We'll have to build a thousand new Yeerk pools just to raise Yeerks for half this number of bodies." Given the world population estimate at the time, this means one average Yeerk pool could contain either an average or a maximum (depending on interpretation) of 2.8 - 3 million Yeerks. In 1996, this was enough for most everyone in the city of Los Angeles and about a third of what was needed to control Los Angeles county. Of course, it's possible that the pool under their hometown served the surrounding area of a smaller city.

Book Quotes

"By now [David's] parents have been taken to a secret, underground facility called a Yeerk pool. It's not a nice place. Picture a sludgy cesspool of a pond the color of molten lead. There are two steel piers leading out over the pond. Hork-Bajir warriors will drag your parents out to the end of one of those piers."
"They will drag them out to the end of that pier and they will kick their legs out from under them and force their heads down into the sludge. And while they are kicking and screaming and calling for help, a Yeerk slug will swim over and it will squeeze into one ear. And it will flatten itself out and squeeze and burrow and dig its way into their skulls, where it will spread around and into their brains. And the Hork-Bajir will yank them up out of the sludge, and they will start to feel that they cannot control their own arms or legs."
Visser flipbook

The flipbook from VISSER shows a boy walking down the infestation pier to a Yeerk Pool to release the yeerk from his ear to absorb Kandrona rays.

--Book 20, The Discovery

As the rest of you know, 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.[...] Ax nodded. <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. > [...]
It was a vast underground cavern. Larger than one of those big sports domes. The stairways and paths emerged from all sides, right about where the upper tier of seats would have been in a sports dome.
In the center of the area was the pool itself, a sludgy, muddy-looking lake that seemed to see the with the mass of Yeerk slugs in it. But that was not the worst of it. Two piers were built out over the lake. One was where the Controllers - human, Hork-Bajir, Taxxon, and other species - disgorged the Yeerks from their heads. Hork-Bajir guards would watch carefully as each Controller knelt at the far end of the pier and held his head down close to the surface of the lake. The Yeerk slug would then slither out of the host's ear and drop with a flat splash into the lake.

-Book 7, The Stranger

"This is gigantic," Marco said. "This isn't just under the school. This is under half the town. Those stairways must lead up to a dozen secret entrances." He shook his head. "Jake, they have this entire area set up with secret passageways. Oh, man. This is worse . . . this is so much worse . . . So much bigger . . . "
I felt the same despair. We were fools. This wasn't some little group of alien bad guys we were dealing with. To build this underground city, these guys had power we couldn't even imagine. That's almost what it was. A city.
There were buildings and sheds all around the rim of the cavern. And we could see yellow Caterpillar earthmovers and cranes at work on the far side of the cavern. They seemed weirdly normal in this incredible place.

-Book 1, The Invasion

Yeerk Pool Entrances

Jake as lizard spying on chapman book 1 the invasion japanese illustration

Jake morphed as green anole lizard, watching as Principal Chapman descends the steps to the Yeerk Pool at an entrance in the janitor's closet of his school. (Japanese version illustration)

  • The Underground - Controllers go up to the counter at McDonalds and order "A Happy Meal with Extra Happy"
  • The Threat - the Animorphs don't go to the main Yeerk pool, but they find a small-scale, portable yeerk pool
  • The Illusion - community center playground tunnel
  • The Revelation - Tunnel which connects to underground facility for docking and repairing Bug Fighters.
  • office building a few blocks from the McQueen building

Small, Portable Yeerk Pools

Eventually, the Yeerks figure out how to create small, portable Yeerk Pools so Yeerks do not have to go to the main Yeerk Pool in order to feed on Kandrona Rays. The small portable pools are discussed in the following books:

  • The Threat - the Animorphs find a small-scale, portable yeerk pool inside a hologram
  • The Reunion - Visser One keeps herself alive with a small, portable Yeerk pool so she does not have to go to the main one, where she might meet her enemy, Visser Three.  When the Animorphs arrive, they see Marco's mother's neck clamped like a collar above the small pool, so she cannot escape while her yeerk is feeding. 
  • The Deception -

Battles at the Yeerk Pool

Animorphs fight at the yeerk pool book 1 The Invasion Japanese Illustration

The Animorphs in their battle morphs fight during a battle at the Yeerk Pool in The Invasion.

  • The Invasion - The Animorphs enter through a long stairway. Jake wants to save his brother Tom, who is a controller and find out what happened to Cassie, who is missing because a Controller-policeman has captured her. The Animorphs see that she will probably be made into a Controller. They get into battle morphs after hearing the screams and seeing the distress of the involuntary human-controllers. They fight Hork-Bajir, Rachel plows through buildings with her elephant morph. Just as Cassie is about to be infested, Tobias rakes a Hork-Bajir's eyes, which allows Cassie to run down the pier, and Jake in tiger morph fights another Hork-Bajir which allows Cassie to free herself. They are all about to escape up the stairs when Visser Three arrives. He morphs an unnamed creature with eight heads, eight legs, and eight arms. The Animorphs run up the stairs as Visser Three uses his morph to hurl fireballs at them. Free humans and Hork-Bajir follow them, but most get burned or recaptured. When Tom (currently uninfested) starts to fight Visser Three with his fists, Jake in his tiger morph joins in and bites down on the Visser's neck. The Visser roars in pain and Jake runs up the stairs. The Animorphs escape and free one human. Tobias escapes but after hiding in red-tailed hawk morph for too long gets trapped as a nothlit in that morph.
  • The Stranger - Marco, Tobias, and Ax have been following the Controller Principal Chapman, and find that there is an entrance to the Yeerk Pool in the dressing room at The Gap. Controllers don't exit the same way - they leave through the movie theater. The Animorphs enter in roach morph by following a Controller and follow a ramp-like entrance. They use their roach senses to go toward the food smell coming from the lunchroom. Just then, a taxxon starts to eat them. Then, the Ellimist stops time to ask them to make a decision about whether to go to a human "zoo" to save some of their species, or to stay on earth and fight. The Animorphs choose to stay and fight. The Ellimist unfreezes time and the Animorphs are returned to being stuck on the Taxxon's tongue in roach morph. They demorph as quickly as they can, then go to battle morphs, fight many Hork-Bajir and Taxxons, and run toward the dropshaft to exit the pool.
  • The Underground - The Animorphs try to enter the Yeerk Pool through the entrance at McDonalds that Tobias has found, but are stopped by a Gleet Bio-Filter. They then proceed to use mole morph to dig their way into the Yeerk Pool, in order to incapacitate as many Yeerks as possible using the insanity-inducing maple and ginger oatmeal. They use a Dracon beam to explode some oatmeal in the yeerk pool and cause part of the ceiling to cave in, then escape by digging themselves out.
  • The Sickness - The Animorphs (without Ax, who is sick) go back to the Yeerk Pool to save Aftran, who is part of the Yeerk Peace Movement and will be interrogated by Visser Three. The Animorphs morph eel and enter the town's water tower, where they plan to slide through the pipes until they reach the Yeerk Pool, but their plan doesn't work because Jake gets nauseous. Soon the rest of the Animorphs except Cassie get sick, so Cassie morphs a yeerk and goes inside Mr. Tidwell, whose Yeerk is part of the Yeerk Peace Movement. They enter through the McDonalds. Cassie goes into the Yeerk pool, and when she hears that Aftran is about to be interrogated, she enters a random host and then steals Aftran. She exits the host, morphs to osprey underneath the Yeerk pool, grabs Aftran, and flies out of the Yeerk pool
  • The Illusion - Taylor tortures Tobias above/in the Yeerk Pool
  • The Prophecy - They go and drain the Yeerk pool.
  • The Mutation - The Animorphs do not go in but do see a ship, the Sea Blade, arise from the Yeerk Pool.
  • The Weakness - the Visser takes Cassie to the Yeerk Pool. The Animorphs enter through a building where ships can enter. They rescue Cassie and later Jake returns.
  • The Arrival - they enter the Yeerk Pool with the Andalites they have just met, who cause damage with their ship, burning through to the Yeerk Pool.
  • The Test - they plan to fry the Yeerk pool with natural gas. Tobias uses taxxon morph to dig to the Yeerk Pool. The natural gas seeps in. Cassie turns it off to save the rest of the Animorphs' lives.
  • The Revelation - Marco calls authorities while posing as a clueless hunter who has just discovered an injured Hork-Bajir he calls a "monster with blades all over it". A Bug Fighter takes 4 minues to arrive to his location. The Animorphs incapacitate the two Hork-Bajir and Taxxon who emerge from the fighter, then acquire the Hork-Bajir, then steal and pilot the craft to enter the Yeerk Pool. The Animorphs raid the Yeerk pool and rescue Marco's mom.
  • The Absolute - there is a temporary Yeerk pool.


  • The official web site at produced a game called the Yeerk Pool Game, in which you play as Jake trying to destroy the Kandrona machine while freeing gorillas and avoiding yeerks.

TV Show

In the TV show the password for the Yeerk Pool under Mcdonald's is Cheeseburger hold the Cheese but in the books it is Happy Meal with extra happy.

In the TV show Jake's Dad opens a health care clinic. The Yeerks take of advantage of this and send Visser Three in morph of Victor Trent to capture the governer. The Governer comes to test the clinic. He is getting a massage and then goes to try Hydrotherapy but Jake was in a office and overheard the plans and tries to stop it. Rachel morphs a lion and attacks Visser Three. The Governer storms out with having a lion in a human health centre. The rest of the animorphs are hiding in the roof above the Hydrotherypy room when the roof caves in and Jake falls into the Yeerk Pool not knowing Jake is a controller they grab and Marco smashes the Kadrona and boils the water on the pool. him and run away. Later in Cassie's barn they were telling Ax about the mission and suddenly Ax attacked Jake. Ax tells them that he is a controller and Jake calls him Andalite Filth they know that Jae is a controller and they tie him up in the barn. It is later known that the Yeerks name is Temrash 114 and was the controller of Tom and was to infest the governer. At the end of the episode the Yeerk dies of Kadronda starvation and falls out of Jake's ear, Jake steps on it and is free. But Tom is reinfested by another Yeerk.



Andalite technology Dome Ship | Escafil Device | Hirac delest | Mirrorwave call | Z-space transponder | Morphing | Mag-Hover Truck | Quantum Virus | Shredder | Gleet Bio-Filter
Yeerk Technology Anti-morphing ray | Dracon beam | Gleet Bio-Filter | Kandrona | Kandrona Rays | Nova-class Empire Ship | Pool ship | Blade ship | Mag-Lev Train | Hunter-Killer Robot | Yeerk Pool | Bug fighter
Ketran Technology Explorer | Time Matrix | Fire-And-Forget Explosive Homer | Uninet | Adge
Other Dropshaft | Ixcila (Arn) | Pemalite crystal | Pemalite ship | Skrit Na Saucer | Hawjabran Colony Ship | Hawjabran Freighter