While the Long Valley Caldera in California may seem like a vast and lush canyon, what lurks underneath has shocked scientists.

Underneath the earth of the 20-miles-long and 11-miles-wide crater lies a whopping 240 cubic miles of semi-molten magma, a new report suggests.

The long dormant volcano was the site of a super explosion 767,000 years ago releasing 140 miles of volcanic material into the atmosphere and devastating the land.

What it left was the 3,000ft deep depression, called a caldera, in central California that still has the potential to explode again.

California's dormant Long Valley Caldera, pictured above, holds 240 cubic miles of magma underneath the surface, according to a new scientific report

The finding is a part of a new report by the United States Geological Survey but they say another explosion is unlikely to occur any time soon

Scientists say there's 240 cubic miles of semi-molten magma capable of leading to another epic explosion like the one that took place 767,000 years ago

The staggering discovery of the magma means that if another eruption were to happen - which experts say is unlikely anytime soon - it would lead to massive natural disaster.

It would eclipse the 1980 explosion of Mount St Helens which belched out just 0.29 cubic miles of volcanic material into atmosphere.

In fact, the valley's storage of 240 cubic miles of magma is enough to fill 400million Olympic swimming pools.

'We can conclude the mid-crustal reservoir is still melt-rich. We estimate the reservoir currently contains enough melt to support another super eruption comparable in size to the caldera-forming eruption at 767 ka,' a report written by scientists from the United State Geological Survey (USGS), the University of California and the University of Rhode Island said.

'This volume and a relatively high melt fraction in no way ensures that the magma is eruptible,' the report added.

The scientists arrived to their conclusion by inspecting the volcano using cutting-edge techniques.

They realized magma may be stewing under the caldera after four strong earthquakes shook the valley area in 1980 and led to evidence of renewed volcanic unrest in the region.

Then they found out that the central part of the caldera was slowly rising.

'Because such ground deformation and earthquakes are common precursors of volcanic eruptions, the USGS has continued to closely monitor the unrest in this region,' the USGS said in a fact sheet.

The magma lies underneath the crater that stretches between Crowley Lake and Mammoth Mountain

The super explosion that took place 767,000 years ago spewed volcanic material and ash that spread as far as Nebraska, Kansas and New Mexico

The prehistoric supervolcano explosion led to the 20-mile-long and 11-miles-wide caldera, shown in pink, that still has magma stirring underneath

'Nevertheless, the long history of volcanic activity in the Long Valley area indicates that future eruptions will occur,' it added.

'Despite 40 years of diverse investigations, the presence of large volumes of melt in Long Valley’s magma reservoir remain unresolved,' the report authors Ashton F Flinders, David R Shelly, Philip B Dawson, David P Hill, Barbara Tripoli, and Yang Shen stated.

California residents have no need too worry though as the geological phenomena take years and many human lifetimes to take place.

After the supervolcano erupted 767,000 years ago, smaller clusters of eruptions have followed roughly every 200,000 years.

Around 100,000 years ago an eruption formed Mammoth Knolls, low hills located north of the Mammoth Lakes.

Mammoth Mountain, a popular trekking spot, is a young volcano itself located on the rim of the Long Valley Caldera.

'When an eruption does break out in the Long Valley area, its impact will depend on the location, size, and type of eruption, as well as the wind direction,' the USGS said.