Mysterious hole in the ground

This entry is part 1 of 5 in the series Mysterious hole in the ground

I came upon the hole pictured in the photos while hiking on a fire road in Mendocino county, (California) a few days ago. Hole was about 1.5 inches (4 cm) in diameter, and lined with a filmy white material which is pictured peeled away in the second photo. I probed the hole with a stick, and it was deeper than one foot. Any idea what made this?  Ben

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