Swimmers were left gobsmacked after a hole appeared in the middle of the water.
The video which was uploaded to Gfycat, was posted on Reddit, where it gained a mass of popularity.
The clip has currently over 150,000 views and has left viewers terrified.
One comment read: “Why is there a giant hole in the middle of the lake where people swim, and why is there not a rope around?”
Another stated: “That looks very unsafe.”
And a third added: “That right there is the portal to Hell.”
However, the giant hole was actually a man-made spillway, also known as a Glory Hole.
The tunnel is used to drain the lake to stop it from overflowing.
Lake Berryessa can hold around 521billion gallons of water before the excess begins to flow into the spillway.
The Glory Hole then drains all the excess water.
In 1997, swimmer Emily Schwalek died after being pulled down the Glory Hole.
It was reported that Ms Schwalek was swimming towards to gaping hole and spent 20 minutes gripping on to the edge before dropping out of sight.
Last year the Lake Berryessa reservoir reached its full capacity for the first time in 11 years.
This comes after a bottomless crater has been discovered in Siberia.
Footage of the crater has been viewed almost 1.5million times on YouTube and the internet has been bombarded with rumours about what made the giant hole.
Another theory suggests global warming caused the explosion.
Speaking to The Siberian Times a government official said it was too early to say what caused the hole but did rule out it being a meteorite.
The official said: “We can definitely say that it is not a meteorite. No details yet.”
A report by Russian news channel Zvezda TV said the dark colour of the crater indicates “some temperature processes”.