Son Doong cave (Photo:Ryan Deboodt)

The newspaper wrote: “Son Doong in Vietnam, the largest known cave passage in the world by volume, which was found in 1990 and only fully explored in the last few years, can be seen by tourists. Only 1,000 visitors are allowed into the caves each year, with each tour taking four days and three nights, and costing £2,245. Those that sign up are rewarded with the chance to experience a cave so big it has its own weather system. At its tallest point the cave can accommodate a 40-storey skyscraper, while at its widest point there’s room to fly a Boeing 747.”