We visit a mysterious sinkhole in Arizona that has befuddled an exclusive group of divers who’ve gotten a glimpse of a strange world at its sandy bottom. For more unusual stories, lists, and offbeat itineraries, check out the Atlas Obscura Explorer’s Guide to the National Parks.

National Parks Week: The Allagash Four

An audio guide to the world’s strange, incredible, and wondrous places. Co-founder Dylan Thuras and a neighborhood of Atlas Obscura reporters explore a new wonder every day, Monday through Thursday. In under 15 minutes, they’ll take you to an incredible place, and along the way, you’ll meet some fascinating people and hear their stories. Our theme and end credit music is composed by Sam Tyndall.