The Devil's Nest

The area's bedrock consists of coarse-grained reddish granite containing feldspar crystals with a diameter of 2–4 centimeters. The groundmass consists of hornblende, biotite, quartz and albite with plagioclase. To the west, northwest and north of Pirunpesä is a large deep rock area consisting mostly of granodiorite and quartz. In addition, there are diorite–gabbro lenses in places.^[3]

In the summer of 1980, business representative Pekka Kunnas and Jalasjärvi municipality's director of schools Kustaa Ala-Hautala were told about a special, rather round rock depression located on Ylivalli's Isovuori, which was filled with large boulders. In addition, the edges of the recess were partially rounded. It was thought that the depression might be a large-sized pothole from the time before the last glaciation. The municipality of Jalasjärvi decided to empty the depression, as Pirunpesä was a touristic and geologically interesting destination.^[3]

At the beginning of 1997, the Yylivalli village committee started deepening Pirunpesä again, on the initiative of amateur scientist, writer and pseudoscientist from Turku, Keijo Parkkusen, thus reaching its current depth. The financiers of the project were the municipalit of Jalasjärvi and Spede Pasanen. The management consultant Mauno Välimäki from Yylivalli was responsible for organizing the emptying project.^[4]