Inside Willamette National Forest in the heart of Oregon is a body of water known as Lost Lake. For a large part of the year, Lost Lake is a fairly unremarkable, shallow, 50-acre pool that serves as a popular fishing hole and hunting area. The rest of the time, Lost Lake isn’t there: Every year, the basin drains through a lava tube at the lake’s bottom, causing the body of water to vanish. The video above, from The Bulletin in Bend, Oregon, shows Lost Lake’s incredible disappearing act.

Lava tubes are a common feature of Oregon’s geology. They form after a volcanic eruption, when flowing lava cools and hardens near the surface while hotter lava continues to flow down below, carving a path as it goes. Occasionally, one of these tunnels will even break through to the surface, as in the case of Lost Lake.

Western Oregon’s rainy months—beginning in the fall—yield such a massive amount of precipitation that the basin fills in at a faster rate than the tube can drain it, and the lake reappears. It freezes over in the winter months, followed by a spring thaw that leads into summer when dry weather results in a (sometimes muddy) meadow. Then the cycle begins again.

Jude McHugh, a spokeswoman for the Willamette National Forest, says it’s hard to predict when the annual drain will occur. While the video makes it seem like a single, continuous flow akin to water in a bathtub, it’s actually a much more gradual process with ebbs and flows that vary from year to year.

“The weather’s so fickle, so if a big rain storm came in tonight and lasted for a week, it would start to fill,” McHugh said. “It doesn’t happen all at once. When the rainfall exceeds the drain, it builds, and when it goes the other way, it sinks. It’s a process.”

This year, a low amount of snowpack and less rainfall have led to the late April/early May drain of 2015.

Lost Lake’s lava tube (best phrase ever?) is about 6 feet wide. It isn’t entirely clear where the water goes once it’s out of sight, but it’s likely that it feeds a subterranean aquifer. McHugh also says it’s difficult to speculate what’s going on in the lava tube when the lake is frozen over.

And if you’re wondering whether there’s such thing as a rubber tub stopper for a lava tube, McHugh told The Bulletin that there have been rogue efforts in the past to plug it, and the U.S. Forest Service has found an array of trash and debris from time to time, likely as part of a poorly-executed and not very environmentally friendly plan to stop the drain. McHugh speculated that if such efforts were ever successful, the nearby highway would probably flood.