Cova de s'Aigua
I'm no geologist but this cave must be a faultline in the rock. It is basically a straight tunnel that runs for many kilometres underground. The cave floor is wet year round (and is home to eels). After squeezing through the narrow entrance the tunnel runs for several hundred metres before the first 'siphon', which comes up to above your neck in water with the cave roof a few centiemetres above the water. Apparently a second siphon follows further on that requires the use of diving equipment to get through, and beyond this are many kilometres more of tunnel. My 'source' claims to have been 6 hours underground without finding any limit.
Very difficult to find. The cave entrance is situtated in the near the base of the western side of the gorge that runs inland from behind the beach of Talis. To find the cave you will almost certainly need to download the attached GPS data and do your research from that! The shortest route to the cave is parking about 1km beyond the Es Migjorn water purification plant and then locating a very overgrown path that zigzags down the side of the gorge. Once at the bottom of the path you can go inland or towards the sea. The cave is about 50m in the inland direction. The entrance is small and well hidden amongst trees and shrubs.