Landslide dams made of debris — boulders and sediment, mainly — are pitted with gaps and piping holes that open and close without warning, making an unreliable structure. Water that goes in also can leach out. It’s a phenomenon seen at similar landslide-formed lakes around the world, Watts said.

At Mountain Lake, when surface water is abundant, the water remains high, and the wet sediment layers keep the leaks in check, Watts said. But when something reduces the surface water going into the lake, piping holes present beneath the sediment drain the water away. Sustained drought can worsen the effect, drying up the sediments and creating more holes.

Watts’ team found four large sinkholes in the deep end of the lake, just below Newport Cottage. Based on that research, in 2013 the endowment’s board of directors mounted an effort to patch the leaks, using natural stone and clay. The operation was part of a $2.25 million effort to modernize and upgrade the resort to bring guests back. The patching worked, Watts said. Monitoring afterwards showed no leaks in the repair site, and the water started to rise.

But by 2014, it was clear the same problems existed in other parts of the lake bed. Watts and his team found other holes, and the first fix never brought it back to full pond.

It wasn’t just the leaks, and the resort owners decided they couldn’t “play whack-a-hole,” as General Manager Heidi Stone said then.