To prevent massive dams from overflowing when heavy rains cause water levels to rise, spillways like this are used to drain water to the stream below. Without a true sense of scale, it looks just like the drain in your bathtub, but from a drone's bird's eye view, the opening to this spillway looks more like a black hole sucking everything in.
Now that California is finally getting a heavy dose of rain, the spillway at Napa's Monticello Dam — referred to as the "Glory Hole" by the locals — is working overtime to manage the water levels in Lake Berryessa. Every second, over 48,000 cubic feet of water is sent down to Putah Creek below. It might not have anywhere near the immense power of a real black hole that can even suck in light, but you do not want to ever find yourself circling this drain.