Watch What Happens When a 'Glory Hole' Opens in Lake Berryessa

For most of the year, the largest lake in Napa County, California, looks like any other lake.

This beautiful reservoir in the Vaca Mountains — created by the hydroelectric Monticello Dam — is, on any given sunny summer day, full of swimmers, fishermen, water skiers, kayakers, canoers, and other boaters. (Not to mention seaplanes, because there's a seaplane base located on the lake.)

But near the dam, when the water is low, there's a strange concrete tower that rises up above the lake's surface. And when the water is high, it turns into an enormous, mesmerizing whirlpool.

The Lake Berryessa 'Glory Hole'

Named after the first Europeans to encounter the place in 1843, José Jesús and Sexto "Sisto" Berrelleza, Lake Berryessa was dammed in 1953 to provide water and electricity for the northern Bay Area. Part of its construction involved designing a spillway (a controlled release valve for excess water).

The spillway design chosen for Berryessa is variously called a bell-mouth, a morning glory, or — most commonly — a glory hole. According to Visit Napa Valley, it's essentially a giant concrete funnel sticking up out of the dam, 75 feet in diameter at the top and 28 feet at the base. When Berryessa's surface level rises over 440 feet above sea level (close to overflowing the dam) it also subsumes the funnel. When water starts to drain out as if from a bathtub, it creates a hypnotizing swirl.

"It's really dramatic to watch," Kevin King, a Solano Irrigation District operations manager told The New York Times in February 2017, when the spillway was active. "I went up there the other day and there were about 15 drones flying around and people taking videos."

Though swimmers and boaters are advised to keep well away from the spillway, the lake remains safe to use even when the lake is over its limit. (The water flowing into the spillway is neither particularly strong nor fast.)

Since the dam's construction in 1950s, The Glory Hole has only been used about two dozen times. Despite increasing droughts and nearby wildfires in the Valley, the lake "is essentially full," Lake Berryessa News editor Peter Kilkus told Travel + Leisure.