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LA's water tunnel runs under a volcano chain that's due

Running 25 miles south from Mono Lake past Mammoth Mountain, the Mono-Inyo Craters are a chain of domes, craters and lava flows built up by a long sequence of eruptions along a narrow crack in the earth's crust in eastern California. The northern stretch, the Mono Craters, began as steam-blast explosions that were later plugged or buried by thick rhyolite domes and glassy lava flows called coulees, while the southern Inyo chain shows open pits, domes and obsidian flows, and the very southern end trails off into the fumaroles of Mammoth Mountain and the small Red Cones. Activity here spans roughly 400,000 years, starting with eruptions that built Mammoth Mountain, continuing through repeated dome-building episodes that formed the Mono and Inyo craters over the last several thousand years, and ending with the uplift of an island in Mono Lake about 250 years ago. Rather than one huge underground chamber, geologists think separate small pockets of magma feed these eruptions, which have recurred every few hundred years for millennia.

The region's volcanic rock has long been useful and striking to people living nearby: Mono Paiutes quarried obsidian here for tools and traded it widely, and a nineteenth-century lumber mill processed timber for the mining boomtown of Bodie. In the twentieth century, engineers cut a tunnel beneath the Mono Craters so Los Angeles could divert water from Mono Lake's tributaries, a project that cost roughly one worker's life for every mile dug and later fueled decades of conservation battles over the shrinking lake. Early visitors, including Mark Twain and John Muir, found the volcanic landscape bleak or strange, but the geologist Israel Russell, who mapped it in the 1880s, admired its colors and symmetry and gave the craters their name. Part of the area gained formal protection in 1984 as the country's first National Scenic Area, though resource use across the whole chain remains managed by the Forest Service.

Because eruptions here have occurred every couple of centuries on average, government geologists treat the Long Valley-Mono Lake region as one of California's active volcanic hazard zones, estimating roughly a one-in-two-hundred annual chance of a new eruption along the chain. Past eruptions have been modest by global standards, but they could still send ash drifting for many miles downwind, choke air routes, trigger pyroclastic flows capable of racing well beyond their vents, or set off mudflows if erupting near snow or lake water. Today the craters and surrounding forests and sagebrush country support wildlife such as mule deer, black bears and marmots, and draw visitors for hiking, skiing, birdwatching and boating around Mono Lake's tufa towers. A scenic highway and visitor centers make much of the chain easy to view, even though reaching the craters themselves usually means leaving pavement behind.

Los Angeles gets water through a tunnel bored beneath the Mono Craters, a chain whose explosion pits on Mammoth Mountain were blasted out within the last 1,000 years.

Read the whole of Mono–Inyo Craters on Wikipedia

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