[strat61caster]

It's probably located further down the "neck" of the fuel tank. If I remember correctly it HAS to be there by law to prevent fumes from escaping the tank, they figured out in the 80's that the fumes that were being emitted while filling up and letting a gas tank "breath" were incredibly damaging to the environment so they came up with systems to trap them.

Now when you fuel up that big rubber donut creates a seal which channels the fumes in your empty tank to the tank under the gas station as your tank gets filled up, same thing happens when the gas station gets their tanks refilled and then the fumes get transported back to the refinery. Not sure what happens to them if they're re-processed into something or just disposed of in a less damaging way to the environment than venting to the air.