A Hole at a Hole-in-the-Ground
A small hole in the bedrock left-over from an ancient gas bubble slowly rising through cooling magma retains a generous reservoir of captured rain water along the crest of the Hole-in-the-Ground crater.
Despite having a diameter of about a mile, a basin 490ft below the surrounding ground-level and a rim that rises from between 110 and 210 feet above, the large maar (volcanic explosion crater) known as "Hole-in-the-Ground" is far enough off of the main path to go completely unnoticed by many visitors to the geologically diverse Fort Rock Basin in Lake County, Oregon.
Located at 43°24?10?N 121°11?54?W, the crater formed during the late Pleistocene, between 13,500 and 18,000 years ago, at which time the Fort Rock Basin was a lake and the location was near the shore. Basaltic magma intruding near the surface flashed ground water to steam, which blew out overlying rock and soil, along with some juvenile material. As material slid into the hole formed, it closed the vent and the process repeated, eventually forming the huge hole...Summarized from Wikipedia - http://en.wikipedia.org/wiki/Hole-in-the-Ground