There is a cave in southern Arizona with a small, man hole like entrance, leading to a relatively small passage. Eventually you come to a squeeze known as the 'selector".

Once (and if) you pop through that, a tiny opening appears in the sidewall. Go through that and you are in wide, spacious walking passage that continues until you reach a pool which feeds a spring in the canyon beyond.

Wind Cave and Jewel Cave in South Dakota are other examples of extensive caves originally reached through very small, squeeze type entrances.

Caving is a great pastime, especially if you are relatively thin, agile, and hyper flexible.....