What's going on with this surreal waterhole in Portugal?

It appears as a kind of magical portal to a watery underworld.

This mesmerising whirlpool, known as Covao do Conchos, has churned away for 60 years in the middle of an otherwise tranquil body of water among the mountains of the Serra de Estrela in Portugal.

And this breathtaking drone footage has provided a closer look at the phenomenon, which is so surreal it's been compared to a mythical sea monster in Homer's The Odyssey.

But what is it?

Anyone searching for a mystical gateway to another dimension might be disappointed from here.

The Covao de Conchos is entirely human-made. It's actually part of a hydro-electric dam system that was built in 1955.

The 1500m-long hole simply funnels water from the lake to the Lagoa Comprida dam at Flores Island.

But with adornments of moss and foliage around the entry to the watery tunnel, you'd be forgiven for thinking it was a natural, if slightly otherworldly, attraction.

Although its remote location means it's gone unnoticed by many most to Portugal, the Covao do Conchos has become a must-see pit stop for adventurous hikers traversing the Serra de Estrela mountain range.