Cave length is a number that can be extracted from survey data. It is quite easy to determine once a cave is surveyed, and although it is not the only way to measure the size of caves, it is the only one that is well defined and internationally accepted.
Length and depth are the only size parameters of caves recorded in international statistics, see below the link to the pages of the Geology and Geography Section of the National Speleological Society. There are different lists for long caves, first an international one, then one for each country. All these lists include mostly karst caves because they are generally longer than other types of caves. In the second place are lava tubes, so there are also lists just for lava tubes. And of course there are other lists for other types of caves, such as gypsum caves or sea caves.
Like other statistics, they change frequently and the position of a cave may change each time the list is updated. However, the caves in first place are so much longer than those in second that it is unlikely that others will catch up with them any time soon.
As I said, there are several ways to measure the size of caves. Alternative representative measures are the area of the floor and the volume of a cave. Both are quite easy to calculate with good software, but they require surveying techniques that are uncommon and vary from country to country. It is necessary to do a full 3D scan of the cave, which is very time-consuming, and the necessary equipment is still very expensive and quite sensitive.
There is a traditional way to use classical surveying techniques. In addition to the normal survey data, which consists of a kind of polygon through the corridors, the distance to the walls and to the ceiling and floor must be recorded. The distance to the walls indicates the width of the passage, which is necessary for calculating the area of the floor. In addition, the distance to the ceiling and floor and the shape of the passage is needed to calculate the volume. These data are not collected in most surveys, and furthermore, the results are extremely inaccurate due to the irregular shapes of most caves. In recent years, international lists of caves by area or volume have been compiled, but the accuracy is quite doubtful.
|Mammoth Cave System, to be exact the Mammoth Cave - Flint Ridge System||Kentucky, U.S.A.||663.050m ||is the longest (karst) cave of the world. The measured length at the moment is 663.050 , more than 663 kilometres, and it has become 75km longer in the last 15 years! This is so much longer than any other cave system that it is very unlikely to find another cave of this length on earth.|
|Jewel Cave||South Dakota, U.S.A.||335.564m ||was the third longest cave for a few decades, after new discoveries of well over 100 kilometres(!) it was the second longest. But it has nevertheless since been pushed back into third place by an underwater cave.|
|Optimisticheskaya||Ukraine||257.000m ||was for a long time the second longest cave in the world, and is now in sixth place, but it is still the longest gypsum cave of the world.|
|Kazumura Cave||Hawai'i, U.S.A.||65.500m ||on the eastern flank of Kilauea volcano is the longest and deepest lava tube in the world. It is 65km long and has a height difference of 1.1km. Strange detail: the cave is never deeper than 20m below the surface! It is 49th on the list of the world's longest caves .|
|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.||271.026m ||is the longest underwater cave in the world. The caves in Mexico are huge cave systems that were formed inside a low limestone plateau during the cold ages. When a lot of water was frozen in glaciers several kilometres thick, the sea level was about 100m lower. The caves drained to sea level at that time and were flooded by the rising sea level. Today they are completely filled with water, both sea water and fresh water from above. As a result, almost all of the long underwater caves are in Mexico, some in Florida with a similar formation history. Sistema Ox Bel Ha is also currently growing at almost 100km per decade, as it is difficult to explore and survey underwater caves, there are still many unexplored areas.|