The length of caves is a number which may be extracted from survey data. It is rather easy to determine, if a cave is surveyed, and despite it is not the only way to measure the size of caves, it is the only one which is well defined and internationally accepted.
Length and depth are the only size parameters of caves, which are collected in international statistics, see the link to the pages of the Geology and Geography Section of the National Speleological Society below. There are different lists for long caves, first an international one, and then one for each country. All those lists mostly list karst caves, because those generally get longer than other types of caves. Second are lava tubes, so there are lists only for lava tubes too. And of course there are other lists for other types of caves, like gypsum caves or sea caves.
Like other statistics they change frequently and the position of a cave may change every time the list is updated. However, the cave on the first place are so much longer than those on the second, it is unlikely others will cath up soon.
As we said, there are different ways to measure the size of caves. Alternative representative mesures are the area of the ground and the volume of a cave. Both are rather easy to compute with a good software, but they require survey techniques which are uncommon and differ between countries. In addition to the normal survey data, which consists of a sort of polyline through the passages, the distance to the walls and to ceiling and floor must be collected. The distance to the walls gives the width of the passage which is necessary to compute the area of the floor. Additionally the distance to ceiling and floor and the form of the passage is needed to compute the volume. This data is not collected in most surveys, and additionally the results are extremely inaccurate, because of the irregular shapes of most caves. As a resulte there do not exist international lists of caves by area or volume, although there might be some national lists.
|Mammoth Cave System, to be exact the Mammoth Cave - Flint Ridge System||Kentucky, U.S.A.||590.000m ||is the longest (karst) cave of the world. The surveyed lenght is at the moment 590,629m , more than 590 kilometers, probably it will reach 600km in a few years! This is so far longer than any other cave system, that it is very unlikely to find any other cave of this length on Earth.|
|Jewel Cave||South Dakota, U.S.A.||218,179m ||was the third longest cave for some decades, but after recent discoveries of several tens of kilometers(!) it is now second. Total length at the moment 218,179m , some more discoveries are likely.|
|Optimisticheskaya||Ukraine||215,000m ||was for a very long time the second longest cave of the world, and is now the third longest. But it is still the longest gypsum cave of the world.|
|Kazumura Cave||Hawai'i, U.S.A.||65,500m ||at the eastern flank of Kilauea volcano is the longest and deepest lava tube in the world. It is 65,500m long and has a height difference of 1,101m. Weird detail: the cave is never more than 20m below surface! On the world long caves list it has place 29 .|
|Painted Cave||Santa Cruz Island, California, U.S.A.||402m||is the longest sea cave of the world, with a length of 402m.|
|Sistema Ox Bel Ha||Quintana Roo, Mexico.||180,039m ||is the longest underwater cave of the world. The caves in Mexico are huge cave systems which developed inside a low limestone plateau during the Ice Ages. When most of the water is frozen in several kilometers thick glaciers, the level of the sea is 100m deeper. The caves developed from the sea level and are today completely filled with water, both sea water and sweet water from above. As a result almost all long underwater caves are located in Mexico, some in Florida with a similar genesis. Sistema Ox Bel Ha is 180,039m long , but as it is difficult to explore underwater caves, there are still many unexplored leads.|