Vietnam’s Son Doong, only open to the public since 2013, is the largest known cave on the planet. With its own climate, still-growing stalagmite formations and rivers, the cave offers adventure junkies a rare experience. February to August, the dry season, is the only time it’s open to tourists, with May offering ideal temperatures. You’ll have to apply to go on a tour, and there is a waiting list, so sign up now. oxalis.com.vn
