坑洞

坑洞是一种垂直类型的洞穴。石灰岩在受水的长期侵蚀下容易形成坑洞。對於探險者來說坑洞更危險，他們需要藉助編織繩等工具才能來到洞底。[1]

墨西哥境內的一個坑洞
小喀爾巴阡山脈的一個坑洞

参考文献
  1. "Your Flexible Friend ... the Ladder", by Dave Irwin in Belfry Bulletin: Journal of the Bristol Exploration Club, Autumn 2007, Number 529, Vol. 36, No. 3
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