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 the 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 172 m 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 158 m.|
|120m||100m||Gua Payau (Deer Cave), Gunung Mulu Park, Sarawak, Malaysia||Deer Cave has the world's largest cave entrance, 100 m wide and 120 m high, and the largest single cave passage in the world, also about 120 - 150 m wide and high.|
|80m||30m||Grotte de Bournillon||The entrance portal is said to be the biggest cave entrance in Europe. Its size is 30 m wide and 80 m high.|
|75m||150m||Gua Niah, Sarawak, Malaysia||The west entrance, also called West Mouth, is an important archaeological site. The enormous cave mouth, 150 m wide and 75 m high, was inhabited over millennia.|
|74m||64m||Teng Long Dong, China||Soaring Dragon Cave has one of the tallest cave entrances, 74 m high and 64 m wide.|
|55m||50m||Grotte de Niaux, Midi-Pyrénées, France.||The huge cave entrance, 55 metres high and 50 metres wide, is at 678 metres above sea level.|
|53m||38m||Gunsight Cave, Carlsbad Caverns, NM, USA.||The entrance of Gunsight Cave at Carlsbad is 53 m high and 38 m wide.|
|50m||25m||Potpecka Pecina, Serbia||The entrance is shaped like a huge horseshoe, 50 m high, 25 m wide in the arch and 16 m wide at the base. It is the largest cave entrance in Serbia.|
|21m||42m||Grutas de Cacahuamilpa, México||The cavern mouth measures 21 m in height and 42 m in width.|
|17m||40m||Grotte de Bédeilhac, France||The 800 m long and up to 80 m high entrance chamber was used as an airplane factory during World War II, in the 1970s a plane was flown into the cave entrance and then started again from the cave.|
|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 20 m wide and some 3 m 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, 33 m wide and 16 m high, leads into the first chamber which is more than 60 m long and 40 m 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 18 m high, 30 m wide, and 105 m long.|
|28m||33m||Green's Cave, MO, USA.||The cave entrance is 28 m high and 33 m wide, and said to be the largest cave opening west of the Mississippi.|
|30m||Peštera Pešna. Makedonia||The biggest cave entrance on the Balkan.|
|10m||27m||Spilia Agia Paraskevi, Crete, Greece.||The entrance to the cave is a large arch, 27 m wide and 10 m high.|
|7.6m||38.4m||Cathedral Caverns, AL, USA.||Claims to have the widest entrance of any commercial cave in the world(?), which is 7.6 m high by 38.4 m wide.|