they live in a large town/mini city in south cali..
but once you think about it, it's was kinda stupid for the yeerks to only have one pool... have to wonder if there where more...
I can't remember where I got this from, but I think it was mentioned (by either K.A or in the books) that the one in their town wasn't the only Yeerk Pool, but it
was the largest and the most developed one that they had so far.
Remember, the Yeerks weren't invading on Earth for very long when the Animorphs began. Maybe about 5-6 years? Edriss (Visser One) and Essam were on Earth for several years making a life for themselves (and building up The Sharing), however, the secret invasion didn't start to fully take off until Edriss presented the Humans to the Yeerks and was promoted to Visser One (with Marco's mom).
Even then, it would've taken some time to assemble and grab the right hosts in power to pull off a project as big as the underground Yeerk Pool, especially in secret. The amount of planning involved in scouting a good place, and then finding the proper funding to bring in construction equipment needed to build something that huge (not to mention the amount of labor time the Yeerks used to actually to build it), would've taken a good 5-6 years for the Yeerks to progress to where the Animorphs saw them for the first time.
Atleast, that's what I've always assumed. It has been a long time since I've re-read
Visser or any of the books.