OK, so it's a hole in the ground. But it's a deep one (23m), caused by millions of years of glacial erosion. You can descend a metal staircase into the cavity, but beware of the devil that lives inside. Alternatively, climb to the top of the rickety 21m-high observation tower and view it from above.
Pirunpesä is 52km south of Seinäjoki. Take Rd 19 to the Jalasjärvi exit, then head southeast for 16km. There's no public transport.