In the mountains of Oregon, there’s a shallow lake just off the highway that disappears once a year during the dry summer months, then reappears during the wetter seasons. It’s called Lost Lake, and its magical vanishing act is thanks to a lava tube.

These geographic features form when streaming lava cools and hardens at the top while the hot insides beneath the surface continue to flow downhill. The tunnel that’s left behind may open up a hole after an eruption or through erosion. Lava tubes ranging from trash-can-sized little guys to subway-tunnel-sized ones you can walk through are scattered across the volcanic terrain of Central Oregon and the Cascade Range.

Several small streams flow into the 0.34-square-kilometer (0.13-square-mile) Lost Lake, and they all drain into one (possibly two) of these large holes on the north side of the lake. Water starts pouring in during the late fall, and it continues throughout the rain and snowstorms. “It fills up in the winter, when input exceeds the rate of draining, and then it goes dry and it’s a meadow,” Willamette National Forest spokeswoman Jude McHugh tells The Bulletin of Bend, Oregon. The hole has been there as long as anyone can remember.

Here’s a very cool video from The Bulletin of Lost Lake funneling down the lava tube drain hole, the lake’s only known outlet:

Lost Lake probably formed about 3,000 years ago, when lava flowing from a volcanic vent blocked a river channel to create a lake, McHugh tells Live Science. It sits atop 12,000-year-old volcanic rock that was filled with bubbles back when it was forming. When the gas escaped into the atmosphere, it left behind pores alongside various cracks and fissures throughout landscape.

We don’t know if the water flowing into the hole travels to an outlet, though McHugh thinks that it likely seeps into the porous subsurface below—recharging the aquifer that feeds the springs on either side of the Cascades. It can take a decade for the water to filter down through all those holes and cracks.

"Here in western Oregon, it pops out at the valley floor and supplies drinking water and important habitat for humans, fish and all kinds of species," she adds. "That water that fell today, there's some kid that's going to be born tomorrow that's going to be drinking it when he's 10." [Via Live Science, The Bulletin]