Despite its appearance, the surreal-looking opening in Portugal’s Lagoa da Serra da Estrela is actually not a portal to any other time or place, but rather a manmade funnel leading into a long tunnel. Though it looks like a natural sinkhole lined with cascading waterfalls, Covão dos Conchos is, in fact, a wonder of engineering.

This lake in the Serra da Estrela mountains was artificially created in 1955 during the construction of the nearby hydroelectric dam. Instead of building a pipeline to channel water between Lagoa Serra da Estrela and Lagoa Comprida, the engineers working on the project decided to drill a tunnel through the mountain to connect the two lakes.

The spillway is designed to provide nearby communities with fresh water, but the fact that no other constructions are present nearby makes the funnel seem more like a part of the lake than a part of an infrastructure project. What’s more, there are plants growing around the edges of the concrete and granite drain pipe, making the giant hole look even more natural. This sci-fi-looking spillway was a little-known secret in Portugal until photos of the hole went viral in 2016.