I googled and found that this is much more common than I thought. It makes me feel bad that this is called an ant nursery. I'm a baby ant killer and their mothers!
Damn! I just read this from a mailman on his blog HERE:
Sometimes during a heavy rainstorm a colony of ants will seek refuge in a rural-type mailbox. Desperate people, finding thousands of ants climbing through their mail, will get out the poison and spray. Then they wind up with a sticky box full of countless ant carcasses. The best way to get rid of the ants is simply to leave the mailbox open. The ants, not liking the light and the vulnerability, will immediately begin to move out, and will soon be gone, eggs and all, leaving no trace that they were ever there.