Britain's third longest cave as well as being its deepest. The deepest point in the cave is at the bottom of the 16 metre deep Gothic Passage sump, so about 275 metres of the depth are traversable by non-divers - also a British Isles record. The trip from the Top Entrance to the Ogof Ffynnon Ddu 1 entrance is the deepest cave through trip in the British Isles at 231 m. It is worth noting that the top few metres of the current Ogof Ffynnon Ddu 1 entrance is mined, as it was originally intended to be a showcave, but the original entrance, which is right next to it, was completely natural. The natural entrance has since been filled with debris, but the through trip depth was actually reduced by about 2 metres as a result of using the artificial entrance instead of the natural one. The approximate depth below the Top Entrance to the lowest point in the cave is about 264 metres, the British Isles record for depth-below-an-entrance. Of that depth, 248 metres are traversable by non-divers, which is also the British Isles record. (The well established total depth of 308 metres was corrected in 2015 to its current value, and the length which had been approximated as 50 km was also accurately measured at the same time as over 10 km more. The altitudes of the entrances and the Gothic Passage sump were also clarified.)
The cave is formed on several levels with in general large breakdown passages at the upper level and a streamway at the bottom level which occasionally becomes a sump level. Passages connecting the levels generally descend steeply and almost all are rift controlled. The cave is decorated throughout with many different styles of calcite decorations of which the most well known are The (~2m high) Columns, with The Trident being the longest stalactite in the UK at 5 metres (there is a longer stalactite in Pol-An-Ionain in the Republic of Ireland). The cave has five entrances, allowing for several through trips (the original entrance is now blocked but would have allowed a deeper through trip, and the resurgence sump is not connected, which would also allow a deeper through trip). Entry is controlled by the South Wales Caving Club and if there is a description for this cave, there's no way it could describe everything, it's far too complicated. There is a virtual guide through a few of the more popular routes. The Survex survey is available online.
Note that the stated length of the cave is the December 2015 total survey length (including vertical components), not the plan length that would normally be quoted for other cave systems. As of February 2017, the plan length of the survey is 56095 metres, but this is missing the lengths of the Birth Canal Series, as well as the sumped sections at Gothic Sump and Dip Sump. Once those sections are added, the plan length should be closer to 60 km. A more accurate length calculation is awaited.