An underwater forest reveals the story of a historic megadrought

A curved tree saw in his gloved hand, a scuba tank on his back, Phil Caterino worked quickly to slice through a pine branch 100 feet below the surface of a small tarn south of Lake Tahoe. Bubbles streamed from the regulator in his mouth, rising through the blue alpine water and green flecks of algae in Fallen Leaf Lake. That autumn day in 1997, Caterino briefly considered what would happen if he accidentally nicked the air hose running to his mouthpiece, or cut his orange dry suit, letting the 39-degree water rush in. "I'd be at the bottom of the lake, dead in about five minutes," he mused.

Having dived some 400 high-altitude lakes over the course of 30 years -- often reciting a protective Washoe prayer beforehand -- Caterino, director of the Lake Tahoe-based environmental nonprofit Alpengroup, doesn't shy away from occupational hazards. He surfaced a few minutes later, branch in hand. Even though the tree it came from had been stewing underwater for 800 years, it still smelled pungently of sap.

This botanic relic is one of several medieval trees, ranging from 68 to 100 feet tall, standing upright at the bottom of the lake. They grew during a 200-year megadrought in the Sierra Nevada between the 9th and 12th centuries, when precipitation in the area fell to less than 60 percent of the average between 1969 and 1992. Fallen Leaf Lake dropped about 150 to 200 feet below its current level, allowing the trees to grow above the lower shoreline. In the wetter years that followed, the lake quickly refilled, drowning the trees and sealing them in a liquid catacomb, safe from insects and fungi in the deep, low-oxygen water. There are also three older trees, which drowned between 18 and 35 centuries ago, standing upright on the lake floor, which suggests that severe droughts struck even further back in time.

The medieval trees' existence adds to the body of research documenting the Sierra Nevada's past megadroughts. Researchers have found stumps of long-dead trees in rivers, lakes and marshes in the region, indicating not one, but two medieval megadroughts -- the other lasting about 140 years in the 13th and 14th centuries, dwarfing the 20th century's Dust Bowl. Such megadroughts are a frightening prospect, and it's possible they could strike again.

John Kleppe, a professor emeritus at the University of Nevada-Reno who owns a lakeside home on Fallen Leaf, accidentally discovered the mysterious climatic archive. For 15 years, his fishing lures bumped against an unknown something in the deeps. "It looked like a fish strike," says Kleppe. "The pole just bent down. I never snagged and never caught anything." Curious, he finally asked Caterino to investigate.

Once Caterino found the first tree, Kleppe began combing the lake for more. He rigged a weighted 150-foot line between the undersides of two boats and slowly scoured the lake. Whenever he hooked a tree, he marked its location and later sent down a camera mounted on a propeller-driven Remotely Operated Underwater Vehicle, or ROV, with a pincer in the front for grabbing samples, and lights mounted on its side. Its footage showed the otherworldly trees with their roots supported by rocks and sediment buildup.

Graham Kent, director of the Nevada Seismological Laboratory, witnessed a surreal underwater scene first-hand from a two-person submersible in 2009. Peering out of the glass orb that encases pilot and passenger, Kent saw lures and fishing lines dangling like tinsel from tree branches, along with tiny single-celled organisms grouped into colonies resembling jellyfish, catching the subsurface light. "It was a bizarre Christmas-tree effect," he recalls. "I was just waiting for Rudolph the Red-Nosed Reindeer to come flying in."

The reindeer failed to appear, but Kent used torpedo-shaped sonar imaging equipment controlled from a boat to rule out the possibility that an earthquake had cracked the lake's bottom, causing water to leak out, or that a landslide had sent the trees tumbling into the lake. The sonar equipment not only helped confirm that drought caused the lake's waters to plummet, it also revealed the ancient shoreline from the past dry spell. His colleagues used tree-ring analysis and radiocarbon dating to calculate the trees' age. This, coupled with a model of how quickly water in the lake evaporates and leaks out into its neighbor, Lake Tahoe, gave the researchers a full picture of the drought's severity and timing.

Knowing that megadroughts hit the Sierra Nevada in the past puts recent dry and wet spells into perspective. In California, the urban and agricultural infrastructure is built on the assumption that winter precipitation for the last 160 years is representative of "everything Mother Nature has to throw at us," says Scott Stine, a professor emeritus at California State University, East Bay, who published a seminal paper in 1994 on megadroughts.

"If we take just a slightly longer view of things, we see that this whole urban and agricultural infrastructure, so dependent on water, is something of a chimera. It is not sustainable in the long term."

• Climate Change