Fonissa is a stream that begins on the village Vrysoules and drains on the east of Kato Loutro, of Xylokastro-Evrostina Municipality, in the Corinthian Gulf. Fonissa stream creates the canyon with the same name, with a total length of 1.5 kilometres.

During the ancient times the stream was named Krios (ram), but was renamed in Fonissa, meaning killer, because it was incredibly dangerous to cross because of its surging torrents.

Its small length doesn't mean its not beautiful. On its length, a hill of limestone interrupts its flow for 1.5 kilometres. In that point, the gorge is very narrow, 2 to 3 metres in width, and it looks more like a cave than a gorge. Very little light reaches the bed and the walls are from rough limestone.

In 1984, a team of experienced climbers crossed the gorge for the first time, and since then, it has been visited many times. It is useful to know, thought, that to cross it from one end to another, a journey of about 3 hours, hiking equipment is needed, as well as basic knowledge of mountain climbing. It has five technical descents, the longest being 18 metres, and it is not considered to be really hard. The best time to visit begins in spring, since the torrent is drying up. A normal visitor, though, can walk in the less rough spots and enjoy the gorge natural beauty with no need for special equipment.

The most narrow and dark points of the gorge are its most interesting, so it is really worth a visit if someone finds themselves on the area. It is good to remember, though, that Fonissa didn't get its name without good reason, and no one should try to cross it when the stream surges towards the Corinthian gulf.