NAPA COUNTY, Calif. — Forged to the Vaca mountains by means of 326,000 cubic yards of concrete is the Monticello Dam. It’s located in a narrow canyon known as Devil's Gate on the southeastern side of Lake Berryessa.

The 304-foot-tall structure can hold back 1,602,000 acre-feet of water, but when water gets too high it spills down the Morning Glory Spillway. 

“Not the 'glory hole.' The official name is the Morning Glory Spillway,” said Melissa Vignau, project manager with Solano Irrigation District.

RELATED: Q&A: How the ‘Glory Hole Duck’ survived its plunge, according to the man who videoed it

Despite what Vignau says, most people call the concrete spillway “The Glory Hole." It’s an odd name for what could have simply been called a big drain.

"Once we reach capacity it starts to flow down the spillway and downstream,” explained Vignau. 

The Monticello dam was built in 1957. Since then, the water has only crested the Glory Hole 24 times. Right now, the water is three feet above the concrete inlet. 

"It’s not an annual occurrence. It only happens when the dam reaches that 1.6 million acres feet."

The Glory Hole's inlet is 72 feet in diameter and the outlet shrinks down to 28-feet wide. Right now, the water is coming out at 3,800 cubic feet per second. Just in case you are wondering, that is enough water to fill an Olympic-sized pool every 23.2 seconds.

RELATED: Duck that plunged down Glory Hole reportedly survived, witness says

Jim Daniels is an engineer with Solano Irrigation. He says the Glory Hole served a different purpose when building the Monticello Dam.

"The spillway was originally built to divert the water to Putah Creek when they were building the dam,” Daniels said.

Below the waters of Lake Berryessa is a flooded town called Monticello. The people, and at least 300 graves, were moved out in 1956. That’s when construction workers stopped diverting the water and built the Glory Hole to an elevation of 440 feet.

“We are in a canyon,” said Daniels. “There is no place to put a spillway, like at Oroville." 

The Glory Hole is a space saver and a number of California reservoirs have one, including Whiskey Town Lake and Trinity Lake.

The gaping hole is definitely a sight to see, but more importantly, when water breaches the Glory Hole, it is a good forecast into California’s water storage.

"We use it for drinking water, for agriculture. There is a lot of prosperity that is possible because there is water in here,” said Daniels.

No one knows how long water will spill into the Glory Hole. In 2017 it lasted several weeks. It all depends on rain and runoff from the mountains.

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WATCH ALSO: Rick Fowler, the man who captured video of the ‘Glory Hole Duck,’ explains how the bird survived

We've all been hearing a lot about it: A duck fell into Lake Berryessa's spillway, also known as the Glory Hole. The whole thing was caught on camera. The man who recorded the video, Rick Fowler, says the duck survived, but ABC10’s John Bartell had some follow up questions.