Onlookers questioned whether they had come across a supernatural whirlpool or a portal to hell as the mysterious hole appeared in Lake Berryessa in California, The Sun reports.

However, although looking like a natural phenomenon, the giant hole is in fact a man-made spillway known as a “glory hole”.

The funnel is used to drain the lake to stop it from overflowing.

The Californian lake can hold up to 1970 litres of water before the excess begins to flow into the spillway.

The lake’s glory hole then operates like a giant bath plug and drains the excess water.

It’s similar to Portugal’s mesmerising Covao do Conchos, another man-made funnel used to remove excess water from a lake, which has become a tourist attraction.

But the glory hole in California also has a dark history.

In 1997 swimmer Emily Schwalek died after being pulled down the spillway. It was reported the 41-year-old was seen swimming towards the gaping hole and spent 20 minutes gripping on to the edge before dropping out of sight.

Last year the Lake Berryessa reservoir reached full capacity for the first time in 11 years, causing widespread amazement.

The original version of this article appeared on The Sun and was reproduced with permission.