A Chinese woman wears her dress and a protective mask as she waits to change after taking pictures in advance of her wedding near the Forbidden City, on April 30, 2020 in Beijing, China. Beijing lowered its risk level after more than three months Thursday in advance of the May holiday, allowing most domestic travellers arriving in the city to do so without having to do 14 days of quarantine. (Photo by Kevin Frayer/Getty Images)

Cave of the Crystals or Giant Crystal Cave is a cave connected to the Naica Mine 300 metres (980 ft) below the surface in Naica, Chihuahua, Mexico. The main chamber contains giant selenite crystals (gypsum, CaSO4·2 H2O), some of the largest natural crystals ever found. The cave's largest crystal found to date is 12 m (39 ft) in length, 4 m (13 ft) in diameter and 55 tons in weight. The cave is extremely hot with air temperatures reaching up to 58 °C (136 °F) with 90 to 99 percent humidity. The cave is relatively unexplored due to these factors. Without proper protection people can only endure approximately ten minutes of exposure at a time.