Cueva de las Manos is actually not one, but several caves in Santa Cruz province, Argentina. The walls are covered in paintings of human hands, human figures and animals created about 9,000 to 13,000 years ago. Almost all of the 800 drawings overlap, creating a multicolored mosaic. The caves were included in the UNESCO World Heritage List in 1999. ¹⁾

Gunung Mulu is perhaps the largest cave complex in the world that has not been fully explored. One of the chambers, the Sarawak Chamber, is 2,296 feet long, 1,312ft wide and over 229 feet high, making it the largest known chamber. ²⁾

Pak Ou Caves, in the cliff walls, lie at the junction of the Ou and Mekong Rivers. They can only be reached by boat. Traditionally, Laotian monarchs visited the caves once a year on Pimai — the Laotian New Year — and left a Buddha statue there. Devotees and pilgrims could also leave sacrificed images there, a practice that continues to this day. The caves were also used by monks as dwellings. Today, both caves contain about 4,000 Buddha statues. The oldest of them are over 300 years old. ³⁾

Mammoth Cave System is the longest cave in the world. It is located in the United States in the state of Kentucky. The total length of its corridors is over 651 kilometers. The cave lies within Mammoth Cave National Park, a UNESCO World Heritage Site. There is a lake at the bottom of one of the shafts, and the corridor systems are crossed by two rivers. It is visited by nearly 2 million tourists annually. ⁴⁾

Optimist Cave is a gypsum cave located near the village of Korolovka, in the Ternopil region of Ukraine. It is the largest cave system of Eurasia and sixth among the caves of the world. It is 146 miles long and the largest gypsum cave in the world. The second-largest in Europe is the Hölloch karst cave in Switzerland, which is about 124 miles long. ⁵⁾