A large and mysterious crater appeared in the Yamal Peninsula of northern Siberia this past July. A few weeks later, two other craters with mysterious origins were discovered relatively nearby. The second crater in Yamal is assumed to have formed in September 2013, with witnesses confident it was a meteorite. There were no witnesses to the formation of the third crater in the Taymyr Peninsula, which was found by shepherds who nearly fell in backward.

The first Yamal crater was initially inspected aerially by helicopter, and soil, water, and air samples were taken near the opening on the surface. There were a variety of explanations thrown around for how these massive holes came to be, from a meteorite impact to weapons testing to aliens taking soil samples. Preliminary investigation found that these holes weren’t man-made, obviously were not extraterrestrial, and were not consistent with a meteorite impact.

The most logical explanation relied on the fact that these craters formed near large methane reserves. This region has been clobbered by climate change and the past few years are the warmest they have been in 120,000 years. It is possible that a pocket of gas in the soil became heated and started building pressure. With the ground thawed more than usual, the gas was able to push out through the surface, popping like a cork. This phenomenon is known as a pingo.

Now that temperatures have dropped and the area has been sufficiently re-frozen, scientists have been able to rappel down into the first Yamal crater in order to explore it up close and take samples from within. With air temperatures at -11˚C (12.2˚F), the researchers climbed 16.5 meters (65 ft) down into the funnel of the crater and onto the frozen lake at the bottom. The lake is predicted to be at least 10.5 meters (35 feet) deep.

Scientists performed radio location tests, peering 200 meters (650 ft) down into the soil. They also took samples of the ice, soil, gas, and air that could help determine if this really was a pingo. The team was led by Vladimir Pushkarev, director of the Russian Centre of Arctic Exploration, who is waiting to see the results from the measurements before speculating on an official cause. However, the team tentatively believes it could have been due to gas hydrates, which are icy crystals of water and gas found in permafrost and under the ocean.

“The main element—and this is our working theory to explain the Yamal crater—was a release of gas hydrates. It turned out that there are gas hydrates both in the deep layer which on peninsula is several hundred metres down, and on the layer close to the surface. There might be another factor, or factors, that could have provoked the air clap. Each of the factors added up and gas exploded, leading to appearance of the crater,” researcher Vladimir Potapov told the Siberian Times. “The crater is located on the intersection of two tectonic faults. Yamal peninsula is seismically quiet, yet the area of the crater we looked into has quite an active tectonic life. That means that the temperature there was higher than usual.”

Once the researchers have a better understanding of the crater’s composition and have a model for how it formed, they will be able to go and investigate the other two recent craters. They then plan to examine satellite data from the last 30 years in hopes of finding other similar structures to study.

Check out these amazing pictures from the team’s expedition into the mysterious Yamal crater taken by Pushkarev:

All images credited to: Vladimir Pushkarev/Russian Centre of Arctic Exploration

[Hat tip: Siberian Times]