Driving up Highway 395 toward Mammoth Lakes is one of the most breathtaking road trips in California. The highway winds through the rugged Eastern Sierra, offering jaw-dropping views of snow-crusted peaks, alpine meadows, and mottled green chaparral plains. But beneath the dramatic landscape lies a hidden danger, an ancient volcanic giant boils beneath the surface.

The Long Valley Caldera in eastern California is an extraordinary geological feature, spanning about 20 miles in length and 11 miles in width. It owes its existence to one of the most dramatic volcanic events in Earth’s history, a super eruption that occurred approximately 760,000 years ago. This event, known as the Bishop Tuff eruption, ejected an estimated 150 cubic miles of molten rock and ash into the atmosphere, far surpassing the 1980 eruption of Mount St. Helens, which released just 0.3 cubic miles of material. The magnitude of the Bishop Tuff eruption resulted in the collapse of the ground above the magma chamber, creating a massive depression known as a caldera. The ash spread for hundreds of miles.

It’s hard to get your head around how big this eruption was.

The Long Valley Caldera is evidence that California is a volcanic state. It sits near one of the nation’s most popular ski towns, Mammoth Lakes. Mammoth Mountain itself is volcanic too, a younger pile of lava domes that rose along the edge of the caldera tens of thousands of years after the giant eruption that created the basin. Geothermal activity, visible in the form of hot springs, fumaroles, and hydrothermal systems, is a ubiquitous feature of the landscape. This activity has made the caldera a hub for geothermal energy production, with the Casa Diablo thermal power plant utilizing its subterranean heat to generate electricity. The energy produced at Casa Diablo is enough to power about 36,000 homes, making it an important renewable energy source for the region.

The surface of the caldera is also marked by the Bishop Tuff, a layer of welded volcanic ash that provides a lasting record of the eruption’s intensity and the pyroclastic flows that reshaped the landscape. Pyroclastic flows are fast-moving, hot clouds of gas and volcanic material that can destroy everything in their path. Here’s a video from Indonesia showing how quickly they move. Often they are considered more dangerous than the lava that pours out of an erupting volcano. For example, pyroclastic flows killed far more people at Pompeii than lava, as the 79 AD eruption of Mount Vesuvius unleashed fast-moving clouds of superheated gas, ash, and volcanic debris that raced down the slopes at over 100 mph, reaching temperatures above 1,000°F, instantly asphyxiating and incinerating thousands, while the slower-moving lava played a minimal role in fatalities.

Geothermal features at the Long Valley Caldera also support microbial communities of thermophilic bacteria and algae, which thrive in the caldera’s hot springs and fumaroles. They carpet the ground for miles, making the surface crunchy and moonscape-like. These organisms not only influence the terrain by contributing to mineral precipitation but also serve as models for studying life in extreme environments, offering analogs for early Earth and potential extraterrestrial ecosystems. Scientists are just beginning to understand how extremophile bacteria live and thrive in deep ocean vent systems.

While the caldera’s formation was sudden and catastrophic — ash spread across much of western North America, as far wast as Nebraska — its story stretches back millions of years. Scientific studies at the Long Valley Caldera have advanced our understanding of volcanic processes, crustal dynamics, and geothermal systems. The Long Valley Caldera sits within the Basin and Range Province, an area of North America characterized by extensional tectonics, where the Earth’s crust is being pulled apart, allowing magma to rise to the surface.

Using seismic tomography, researchers have mapped the magma chamber beneath the caldera, revealing a layered structure with a partially molten zone capped by solidified magma, like a hot dish of au gratin potatoes. A 2023 study published in Science Advances, explains the periodic episodes of unrest at the caldera and offers scientists a baseline to understand potential future activity. Even after the formation of the caldera, volcanic activity continued in the area. The nearby Mono-Inyo Craters area has seen eruptions as recently as 600 years ago, a blink of an eye geologically speaking.

Another place where the region’s volcanic activity can be experienced firsthand is Horseshoe Lake, where carbon dioxide continuously seeps from the ground, suffocating tree roots and causing a vast die-off of trees. The result is a barren, almost ghostly landscape of skeletal trunks and barren ground. It’s a fascinating place to explore, though the posted warnings are hard to ignore. Signs around the area caution visitors about the danger of carbon dioxide — odorless and colorless — that can accumulate invisibly to levels that are dangerous, even deadly, if inhaled.

The caldera remains active. Ground deformation studies, using GPS and InSAR technology (satellites), have tracked uplift in the caldera’s floor, offering critical data on magma movement and hydrothermal activity. In a 2016 study published in Geophysical Research Letters, researchers linked changes in uplift patterns to deeper magmatic processes, and scientists are constantly monitoring the area should activity increase. In 1980, a series of magnitude 6 earthquakes occurred the southern margin, drawing the attention of United States Geological Survey (USGS) volcanologists. The earthquakes were accompanied by noticeable uplift in the caldera’s floor, a sign of magma movement beneath the surface. One spot of particular interest is a resurgent dome, just outside Mammoth, where the floor of the caldera has slowly lifted upward as magma pushes from below. Over the past several decades the ground there has risen and fallen by several feet as the volcanic system beneath Long Valley expands and contracts, although scientists generally say that the level of volcanic unrest at Long Valley has declined since the early 1980s.

As a result of the 1980s studies, the town of Mammoth Lakes took proactive measures to ensure public safety. Local authorities constructed new road, an “emergency evacuation route,” to serve as an escape in the event of a volcanic eruption or other natural disaster stemming from the Long Valley Caldera. After local businesses and residents complained that the name was scaring people away, in an act of geological marketing, it was changed to the Mammoth Scenic Loop to emphasize the area’s beauty and appeal. That said, the USGS intensified its monitoring efforts, implementing a color-coded alert system to communicate volcanic activity risks.

Beyond its scientific significance, and the small, but not insignificant potential for catastrophe, the Long Valley Caldera is a wonderful destination for outdoors folk. Numerous hot springs dot the landscape and are immensely popular among tourists and residents. There are amazing opportunities to hike and fish during the summer and superb skiing in the winter months. So, despite the dangers, which I don’t want to overstate here, it’s an awesome place.

Of course, Mammoth Mountain does has its geologic dangers—like I said, it’s a volcano. On April 6, 2006, three members of the Mammoth Mountain ski patrol tragically lost their lives after falling into a volcanic fumarole near the summit. The incident happened while they were conducting safety operations to secure a snow-covered geothermal vent following an unprecedented snowfall. If you’ve ever skied Mammoth before, there is a distinct sulphurous smell around the Christmas Bowl ski run at Chair 3 near McCoy Station.

Standing in the middle of the Long Valley Caldera, it’s hard to grasp the scale of what happened here. It’s an extraordinary place to spend time, especially at night, when the Sierra sky is speckled with stars and the valley becomes silent, except for the whisper of the occasional wind gust. The ground beneath your feet is the result of an astonishing geologic event a long time ago, but it’s still active. There are few places quite like it in California. And for a state filled with volcanoes, that’s saying something.