Vrelo Cave on the Treska River in Matka Canyon, near Skopje, North Macedonia. Some speculate that it is the deepest underwater cave in the world.

Matka Canyon is easily visited during a day trip from Skopje, North Macedonia using public transportation. It offers miles of hiking along the deep river formed by damming of the Treska River, as well as trails to three historic monasteries. A restaurant and hotel offer additional amenities, including boat trips to Vrelo Cave, purported to be the deepest underwater cave in the world.