California: There is a mysterious hole in Lake Berryessa in California. It is not a supernatural whirlpool, a demon's mouth or a portal into hell or a fourth dimension. The creepy thing probably won't suck you into it either.

It is just a really big drain called a spillway. And once you see it full of spiralling water, it is hard to take your eyes off it.

For the first time in a decade, January and February have brought so much rain that the lake in the Napa Valley area north of San Francisco has maxed out its water capacity. To prevent flooding, this 22-metre-wide concrete funnel is sucking down the excess. Hundreds of locals have gathered to watch the show and take photos and videos since it started on Friday. And the show isn't over. More storms are expected toward the end of the week, and the flow may continue for up to two weeks.

"I went up there the other day and there were about 15 drones flying around and people taking videos," said Kevin King, an operations manager at the Solano Irrigation District, which oversees the day-to-day activities at the dam. "It's really dramatic to watch."

This is what happens when engineers build stuff to try to prevent dams from flooding. Recently, California's dams have gotten a lot of attention after 180,000 people were forced to evacuate their homes because of problems with the Oroville Dam's spillways. So far, there is no worry about flooding at Berryessa.

Spillways come in many shapes and sizes. The one at Berryessa is of the "bellmouth" persuasion, which is also called a morning glory, plug hole or a glory hole, as the locals refer to it. The structure works a lot like the hole in the side of your sink or bathtub, which keeps water from spilling out onto the floor if someone leaves the faucet running. Only with a reservoir, it works when the rain won't stop, as has been the case in this area, which has seen four wet storms known as atmospheric rivers so far this year.

When water rises more than 134 metres above sea level in Lake Berryessa, it spills over the lip of the morning glory, funnels down the cone and exits into Putah Creek, on the other side of the Monticello Dam. The lake can hold about 521 trillion gallons of water before needing the morning glory to prevent floods.

On Tuesday, water was flowing into the hole at around two million gallons a minute (7.6 million litres a minute), which is just a small fraction of how fast it can get.

Powerful as it looks, the spillway probably wouldn't pull you down to the netherworld if for some reason you found yourself in the lake. Although it could still be dangerous (and there are safety barriers and buoys to keep people away), you could probably swim away from the current.

"It's a lot of water, but not a lot of velocity," King said.

In the world of spillways, bellmouths like this one are uncommon, King added. Engineers pick them because they can spill a lot of water at one time and still maintain control, said Sarah McBride, a public affairs specialist at the US Bureau of Reclamation, which owns Lake Berryessa's morning glory. There are others spillways of this type around the world, including at the Shing Mun Reservoir in Hong Kong, Hungry Horse Dam in Montana, Nekogahora Pond in Japan and the Ladybower Reservoir in England.

Although the water has risen about 9.1 metres since January, there is currently no danger of flooding at Lake Berryessa, according to King and McBride. But McBride urges people to stick to trails and avoid the lake's shoreline, where mudslides and falling rocks are a concern.

To see the morning glory in all its glory, park your car at the lot near Monticello Dam and walk over to see it.

"You can definitely hear it as it's cascading," King said.

The New York Times