Inside Son Doong cave in Quang Binh province.

The magazine said the incredible depths of the world’s largest cave lures in travellers who want a true adventure. But the arduous journey into this subterranean site makes booking a trek essential.

The major tour operator that takes travellers inside, Oxalis Adventure Tours, says on its website that less than 300 spaces remain for 2019 tours. If you want to make it happen book soon – or get on the waiting list for 2020.