Scientists have pulled back the curtain on one of nature’s most bizarre disappearing acts, the annual draining of Lost Lake in Oregon’s Mount Hood National Forest. Every year, the lake, which gets fed in the spring by melting snow, vanishes down a sunken hole on the lake’s north side and becomes a grassy meadow. The curious phenomenon caught the attention of social media users this week after a video of Lost Lake was posted online by the Bulletin. Where exactly the water goes has long been a mystery, but the most likely explanation involves the landscape’s violent, volcanic past.

The hole is actually a collapsed lava tube, a relatively common geological feature in the region. Lava tubes were formed when streams of lava cooled and solidified on the surface but continued to flow underground, eventually leaving large tunnels in its wake. “When the supply of lava stops at the end of an eruption or lava is diverted elsewhere, lava in the tube system drains downslope and leaves partially empty conduits beneath the ground,” the U.S. Geological Survey says.

An opening in one of those channels is the hole seen in the video of Lost Lake. After the water disappears into the earth, it likely spreads to other springs throughout the national forest and probably refills an aquifer in Oregon’s Cascade Range. Scientists say it takes seven to 10 years for the water to reach the valley floor, the Huffington Post reported.  

Plugging up the hole to try and keep the lake year-round isn’t a good idea, said Jude McHugh, spokeswoman with the Willamette National Forest. “If anyone was ever successful at plugging it -- which we’re not sure they could do -- it would just result in the lake flooding, and the road,” McHugh told the Bulletin. She said people have previously attempted to stop the lake from draining by filling the hole with debris, including car engines.

At 11,239 feet tall, Mount Hood is the highest point in Oregon. The volcanic peak has been dormant since the early 1900s, the Great Outdoor Recreation Pages say.