You might not think about caves in the same breath as you do the deep ocean or outer space, but you probably should. There are approximately 70,000 caves in the United States alone, but the vast majority are inaccessible to the public. That means rare, delicate ecosystems have developed for tens of thousands of years in complete isolation from human contact. That is, until cavers travel deep underground through impossibly small spaces to find them. Join the Endless Thread team as we dive into the claustrophobia-inducing world of caving. This episode was originally published on September 4, 2020.