Near the area of Konistra and the settlement of Lakos is Kolethra, one of the largest springs in Evia. It is known as Koletra of Agios Vlasis due to the settlement that is located about 12 kilometers south of Kymi and 137 kilometers from Athens.

The spring stands in the shadow of an imposing rock, in which there is an underground cave, and is part of a underwater complex consisting of smaller springs, torrents and sinkholes. According to the researchers, this natural network collects groundwater from Mount Mavrovouni, which rises to the west at a height of 1,190 meters and is the eastern end of Dirfyos. The water of the spring irrigates 11 villages in the surrounding area.

The amount of spring water depends on the season, as well as the snowfall and rainfall each year. In winter and spring the waters swell, creating a beautiful lake that feeds a rapid torrent that flows south and reaches after about 3 km the river Manikiatis. In summer and autumn the water level drops, causing the lake to shrink or even dry up (usually in July and August), leaving an empty basin about 10 meters deep.

The point is an attraction for nature lovers, while the cultural association of the village of Agios Vlasios has formed a small recreation area at the entrance of the spring, where you can enjoy nature in all its glory. The small path (300 meters) that starts from there will lead you to the canals that supply the adjacent water mill that is in operation.

It is one of the largest caves in Greece, as it has been explored underwater το a length of 1.5 km by previous cave diving expeditions. It remains unknown how far the cave reaches underwater. Diving by the Addicted2h2o team in the cave of Kolethra has taken place on September 10, 2017, on July 7, 2018, with the most recent dive on July 31, 2021 as seen in the video below.