California's Long Valley Caldera is one of the world's biggest calderas, measuring 20 miles long by 11 miles broad and up to 3,000 feet deep. The Bishop tuff, a welded tuff that characterizes the area, was produced 760,000 years ago when a cataclysmic eruption ejected hot ash that eventually cooled and became the Bishop tuff.

Upon eruption, the ash went eight miles into the air, with deposits reaching as far east as Kansas.

Despite the potential for total disaster if Long Valley erupts, nothing is written about it. Instead, the focus is on Yellowstone, a supervolcano located hundreds of miles to the northeast.

Long Valley, though, may be on the verge of exploding, according to the Science Channel.

Secrets of the Underground

'Secrets of the Underground,' a 2017 documentary, looked at the supervolcano and its recent activity.

"There are worrying signals of probable volcanic activity," said Rob Nelson, a scientist, and the show's narrator.

"And hints are pointing to an impending eruption strewn over this valley - the location of North America's second-largest explosive volcanic eruption."

Related Article: Yellowstone Eruption 630,000 Years Ago Opens New Geological Mysteries

A Possible Threat

Even though the recent eruption from Long Valley isn't as large as past ones, it still poses an "existential threat" to the millions of people who live nearby.

The Science Channel investigated a section of the valley and discovered many clouds of smoke pouring from beneath the ground.

Using InSAR data that has been monitoring the region for the last 20 years, geophysicist Jared Peacock pointed out a worrying aspect of the caldera that might portend problems.

InSAR is a remote sensing method that employs a laser to concentrate a beam of radiation on a target, bouncing back to a sensor on an antenna, providing a comprehensive map of a region.

One of the most concerning sites in InSAR was near Mammoth Lakes, a hamlet in the Sierra Nevada highlands.

"Right here in the center, you see there's a resurgent dome," Mr. Peacock said, pointing to a map made from the data.

A blazing-hot red spot is depicted just beneath the Earth, indicating the presence of magma.

"Something beneath it is pulling it higher," Mr. Peacock continued.

Studying Volcanic Activities

Mr. Peacock and Mr. Nelson set up a pair of sensor pipes immediately above the site. The InSAR data revealed the resurgent dome and checked for signals of problems deep below to see if the Long Valley Caldera was coming back to life.

The pipes assisted the two scientists in detecting changes in the Earth's magnetic field, allowing them to establish whether any liquid existed underneath.

They observed enormous volumes of liquid beneath the domes' surface during the experiments, indicating volcanic activity.

This activity, however, was not centralized, which is grounds for concern. Instead, it was scarce and dispersed.

"We can declare certainly that there is no huge magma chamber beneath," Mr. Peacock added, "but smaller satellite magma chambers are surrounding the area."

"The occurrence of huge amounts of melt in Long Valley's magma reservoir remains unanswered despite 40 years of varied research," the paper says.

The Long Valley Caldera reservoir is predicted to have "significant melt characteristics," with a volume of more than 240 cubic miles (1,000 cubic kilometers).

This melt might be hot enough to burn liquid rock in around 27% of cases.

Long Valley last erupted 100,000 years ago, according to the United States Geological Survey (USGS).

Also Read: One of the World Largest Mass Extinctions May Have Been Triggered by Volcanic Winter

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