THE BATS WERE in a frenzy. At first it was just a single creature, flashing past my face as seven of us, wearing life jackets, helmets and headlamps, bobbed in a shadowy river inside a 3-million-year-old cave in north-central Vietnam. As we swam deeper into the darkness, into a watery chamber the size of a basketball arena, our beams revealed scores of bats zipping around us from all directions, feasting on constellations of tiny white bugs orbiting our illuminated heads. Cave swimming isn’t for everyone.

I was halfway into...