Seen above is the Morning Glory Spillway in Napa County’s Monticello Dam. The dam creates Lake Berryessa, a 1.6 million acre-foot (about 2 million cubic meter) man-made body of water which serves as a recreational site featuring swimming, boating, fishing, hiking, and other outdoorsy activities. To make sure that the surrounding roads and area do not flood, there’s a spillway. When the water level goes over the spillway’s lip, it instead ends up flowing, rapidly, down the hole and into the drainage basin below. At the base is a hydroelectric power plant whose three generators provide electricity to the San Francisco area.

Which is nothing special. There are many spillways out there, designed to do basically the same thing the Morning Glory Spillway does. What makes the Morning Glory Spillway particularly interesting is its size. An Olympic-sized swimming pool holds roughly 88,000 cubic feet (or about 2,500 cubic meters) of water. The Morning Glory Spillway can drain that in under two seconds.

The spillway is effectively a very large funnel. Its mouth is 72 feet in diameter (seen above, dry) and runs 400 feet, tapering to a diameter of about 28 feet at the bottom opening. When it is dry, it serves a secondary purpose — as an illegal skate park for skateboarders, as seen here. The only problem? Even when the spillway itself is dry, the drainage basin typically is not, so the skateboarders need to take a raft to get there.

Bonus fact : There is a very rare cloud formation known as the Morning Glory cloud. It is a roll cloud — a horizontal, tube-shaped one — which is only known to appear regularly in the Gulf of Carpentaria north of Australia. Seen here, these clouds can be 1,000 kilometers long, a kilometer high, and move at 60 kilometers per hour. The cause of these clouds is unknown.

From the Archives: In Super Mario’s World, Clouds Are Bushes: It has to do with clouds, like the bonus fact does. It’s also the only Now I Know which is (almost) entirely an image. It is from the very, very early days — issue three, I think — when I was still trying to figure out what I was doing here. Not to say that I’ve figured it out yet, but I won’t be doing that again.

Related: A lit waterfall spillway. Not sure what it does or why it costs over $130 — nor why it looks like a dustpan.