Cave entrances are often much bigger than the cave behind. The explanation is simple: they are in the reach of outside weather conditions, and thus subject to weather related erosion. There is sun heating the rock, cold winds and frost cooling the rock, the continuous temperature change makes the rock brittle. Water from the humid air of the cave sits in cracks in the rock, if the outside freezes in winter the water becomes ice, expands, and destroys teh rock from inside.
To determine the size of the entrance portal, there are actually only two numbers which can be measured, the height and the width of the portal. The portal itself is defined easily, its the cross section of the cave where the cave ends, thats exactly the line where rain from above would end.
Through history, with every huger cave entrance discovered, the formerly biggest cave entrance became the second biggest, and so on. Unfortunatly the term "biggest cave portal" stays, once coined, almost undefinitely.
|250m||90m||Cueva del Fantasma, Venezuelan Guayana||Formed by the collapse of an enormous gorge, this cave entrance is big enough for two helicopters to land at the same time.|
|172m||Gruta Casa de Pedra, Brazil||The Gruta Casa de Pedra (House of Rock) in Brazil has an entrance which is 172m high. It was the highest known cave portal for many decades.|
|158m||35m||El Arco del Tiempo del Río La Venta, Estado Chiapas, México||This short through cave in the Cañón del Río La Venta is for unknown reasons advertised as one of the largest natural stone arches in the world with a height of 158m.|
|120m||100m||Gua Payau (Deer Cave), Gunung Mulu Park, Sarawak, Malaysia||Deer Cave has the world's largest cave entrance, 100m wide and 120m high, and the largest single cave passage in the world, also about 120 - 150m wide and high.|
|80m||30m||Grotte de Bournillon||The entrance portal is said to be the biggest cave entrance in Europe. Its size is 30m wide and 80m high.|
|75m||150m||Gua Niah, Sarawak, Malaysia||The west entrance, also called West Mouth, is an important archaeological site. The enormous cave mouth, 150m wide and 75m high, was inhabited over millennia.|
|74m||64m||Teng Long Dong, China||Soaring Dragon Cave has one of the tallest cave entrances, 74m high and 64m wide.|
|53m||38m||Gunsight Cave, Carlsbad Caverns, NM, USA.||The entrance of Gunsight Cave at Carlsbad is 53m high and 38m wide.|
|50m||25m||Potpecka Pecina, Serbia||The entrance is shaped like a huge horseshoe, 50m high, 25m wide in the arch and 16m wide at the base. It is the largest cave entrance in Serbia.|
|21m||42m||Grutas de Cacahuamilpa, México||The cavern mouth measures 21m in height and 42m in width.|
|3m||20m||Porth yr Ogof, Wales, Great Britain.||Porth yr Ogof is the largest cave entrance in Wales and one of the largest anywhere in the United Kingdom. It is nearly 20m wide and some 3m high.|
|16m||33m||Smoo Cave, Scotland, Great Britain.||Smoo Cave has the largest entrance of any sea cave in the British Isles. The huge entrance, 33m wide and 16m high, leads into the first chamber which is more than 60m long and 40m wide.|
|Hidden River Cave, KY, USA.||Kentucky's largest show cave entrance.|
|18m||30m||Peak Cavern, Great Britain.||The entrance of Peak Cavern (The Devil's Arse), Castleton, Derbyshire, is the largest cave entrance in the British Isles. It is 18m high, 30m wide, and 105m long.|
|28m||33m||Green's Cave, MO, USA.||The cave entrance is 28m high and 33m wide, and said to be the largest cave opening west of the Mississippi.|
|10m||27m||Spilia Agia Paraskevi, Crete, Greece.||The entrance to the cave is a large arch, 27m wide and 10m high.|
|7.6m||38.4m||Cathedral Caverns, AL, USA.||Claims to have the widest entrance of any commercial cave in the world(?), which is 7.6m high by 38.4m wide.|