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The Well of Merro or just Merro is a karst chasm, a sinkhole, which is home at the bottom to a pond about 20 m in diameter. With its depth of 392 meters it was the deepest in the world, before Zacatòn in Mexico. The hypothetical bottom has been plumbed with an underwater robot (thanks to firefighters) but all probes lowered into the well until now have not found the real bottom.

It is located within a protected region and is an ideal habitat for certain types of plants and animals, the most significant are some fish and crustaceans such as Niphargus cornicolanus you would find only in this well at a depth of 70 meters.