The massive Morning Glory Spillway in the Lake Berryessa reservoir in northern California was originally designed to funnel away water after unusually big storms, and was only expected to flow a couple of times per century when heavy rains filled the lake to capacity.

Key points: At 22m wide and 75m long, the spillway is the largest of its kind in the world

At 22m wide and 75m long, the spillway is the largest of its kind in the world It is draining water for the second time in two years after weeks of rain drenched northern California

It is draining water for the second time in two years after weeks of rain drenched northern California Cordoned off from swimmers and boaters, it can sink water at 1,360 cubic metres a second

But recent dramatic video and photographs show the circular spillway, known by locals more colourfully as the "glory hole", flowing for the second time in two years, after the brimming lake became too big for its banks.

"We had a decent amount of rain overnight and we knew it was close to the lake being full," local resident Sandy Storck told SFGate.

"It's always exciting to see it spill over," she said.

The compelling sight of the 22-metre-wide, 75-metre-long drain in full flow after record recent rains in the area is also reportedly inspiring locals to wax lyrical about what it reminds them of when the water's really pouring through it.

It looks a bit like a giant plughole in a bath, but some apparently think of it as a beautiful inverted fountain. ( Photo: Doug Letterman/Flickr )

According to the Guardian, some said water coursing through the spillway's "bell mouth" looked like a toilet being flushed.

More generous descriptions were that it resembled a giant whirlpool or a beautiful inverted fountain in the lake, which is a popular haunt for boaters and swimmers.

It could also simply be described as a really big plughole.

Whatever it's called, the Morning Glory Spillway is one of the largest drains of this kind in the world — and is an impressive sight when the water is high.

Cordoned off from boaters and swimmers

When the water gets to a certain level after enough rain falls, the glory hole can gulp lake water at a mighty rate of about 1,360 cubic metres per second.

The water then spirals down, travelling horizontally for more than 600 metres before draining into Putah Creek nearby.

But while video footage seems to show the flow of H2O creating a strong pull toward the spillway's maw, the movement of water down its funnel actually doesn't create that much suction, the Guardian said.

It is not without its dangers, though, and is cordoned off from boaters and swimmers by a buoy line, which local authorities reportedly check regularly.

It is apparently illegal for people to approach the spillway, too, and local laws are in place that prohibit boats from travelling near dam walls — like the wall of the Monticello Dam nearby.

It is illegal for people and boats to come close to the circular spillway. ( Flickr: Doug Letterman )

Unfortunately, this didn't stop one woman disappearing into the hole in 1997.

The San Francisco Chronicle reported at the time that Emily Shwalek, 41, had been seen swimming toward the spillway about 6.30pm one Sunday evening.

She reportedly gripped the edge of the hole for about 20 minutes, witnesses said (presumably at a time when the water in the lake was not so high), and then dropped out of sight.

Her body was found about three hours later.

There had not been a documented case of anyone else falling through the glory hole, a local irrigation authority spokesperson said.

Skateboarders use spillway as a ramp

When the lake is full, water flows into it then travels horizontally for 600 metres before emptying into a creek. ( Photo: Daniel Clanon/Flickr )

According to the National Geographic, the glory hole spillway acts as a drain for Lake Berryessa, a man-made lake created when the Monticello Dam was built in the Napa Valley, northern California in the 1950s.

The Monticello Dam reportedly provides irrigation and drinking water for around 600,000 people in the area.

It can hold 526 billion gallons before the glory hole streams into action and excess water needs to be drained away.

And when it's not so full, in the dry season, skateboarders reportedly use the exit of the giant spillway as a ramp.