Demon of Paasvesi

The Demon of Paasvesi/Paasselkä (Finnish: Paasveden/Paasselän piru) is a mysterious light that glides on the surface of lake Paasselkä (formerly known as Paasvesi) in Southern Savonia. Stories of the demon or piru go back at least 300 years. Sightings of it still happen in the modern day. The name also appears in plural (pirut = demons). Paasselkä lake is originally a crater, created by an impact 230 million years ago. It is round and has no islands.

Description
The light is usually described as white or red, and it appears around the middle of the lake. It can move long distances suddenly or stay put. It has been said to grow brighter or disappear at times, and change place in a blink. When it moves, there is a trail after it, according to one eyewitness.

The oldest known story about the light is from the 18th century. At the time, the lake was very close to the border of Sweden and Russia. A Savonian man had gone fishing to the non-Savonian waters, only to be chased away. A bright light had appeared from the water and guided the man to safety. Border areas of any kind had been considered to be sacred, and places to avoid. Those who moved borders illegally might have ended up haunting the site of the original border. Nowadays, the area is at the border of the regions of Southern Savonia and Northern Karelia in Finland.

According to locals, the light appears less often now than it did in the 20th century.

Theories
Theories about the cause of the phenomenon are plenty, but there is no certain answer. Apart from being called pirus, the lights have also been described as virvatuli (will-o'-the-wisp), or souls of drowned Russian sailors. A more modern but still supernatural theory suggests that the lights come from aliens or UFOs. The site is certainly not the only place in the region where strange lights have been seen: sightings of lights at nearby Rääkkylä caused a UFO search craze in the region in the 90s.

A scientific approach suggests that the lights are caused by methane or some unknown chemical reaction brought from outer space by whatever crashed in and created the crater in the first place.