This strange hole in the water is not an optical illusion

When the heavens open in the eastern part of Napa Valley in California, a water portal to hell may open.

If the water level in the Lake Berryessa reservoir at the top of the Monticello Dam rises too high, the excess water begins to swirl into a large hole, as if carved into the surface of the lake.

Known to locals as the "Glory Hole", this hole is not really a hellmouth, although the whirlpool looks like it. This is a spillway built by engineers in the 1950s. The hole is an alternative to the more classic side chute, which is used to control the flow of water from a dam or dam.

But in a narrow gap between the rocks, such as the Devil's Gate, where the Monticello Dam is located, the side chute would have nowhere to go. Instead, the builders settled on a big old drain, the same as in your bathtub or sink.

This design is known as the "bell mouth" (completely rhymes with "hell's mouth"); they have been used to control water levels in several other dams around the world.

However, the "Glory Hole" in Napa is one of the most famous, because it leads to a very interesting spectacle. In a particularly rainy 2017, hundreds of people gathered on a concrete ledge to stare open-mouthed into the gaping abyss, The New York Times reports.

In 2019, after another season of heavy rainfall, the "Glory Hole" started working again, this time attracting thousands of curious viewers.

In the same year, the unfortunate cormorant made headlines when he was filmed being sucked into a chute (according to some reports, he survived).

Fortunately, in our days, the same accident most likely would not have befallen a human swimmer. Firstly, the reservoir is fenced and prohibits swimming and boating. Secondly, most people can easily swim against the speed and strength of the whirlpool, even when it is in its most powerful state.

The only case of human death in the reservoir was recorded in 1997, when a woman swam out onto a cement structure. She clung to the edge for 20 minutes, but rescuers arrived too late. Her body was found a few hours later.

Although the whirlpool may look intimidating from above, those who work on it say that the speed of water movement is not so great, it just flows down at the same time.

Every second, the 22-meter-wide (72-foot) and 75-meter-long (245-foot) sewage pit is capable of swallowing about 1,360 cubic meters of water (48,000 cubic feet). After the initial fall into the hole, the water enters a narrower pipe, which, after more than half a kilometer of horizontal movement, flows into the nearby Putah Creek.

Today, the "Glory Hole" is well protected so that no one accidentally gets hurt. When it rains, tourists often line up along the fences to watch the hole in action.

In the 1950s, when engineers were building the Glory Hole, they thought it would only be used once every 50 years in extreme situations. However, since the beginning of the century, the whirlpool has opened three times.

What was built for an unlikely scenario has now become more of a seasonal sensation - another reminder that the world's climate is changing.