The Serra da Estrela is the highest mountain range in continental Portugal. In this area rests a body of water with a rather peculiar circular landmark near one of its banks. This landmark is a giant hole that looks like it feeds directly into the gates of hell. 

Called the Covao dos Conchos, this hole is actually a man-made dam that is part of Portugal’s hydroelectric system. The water travels through a system of tunnels until it reaches Lagoa Comprida dam.