Russian researchers are beginning an "urgent" investigation into why more mysterious craters are appearing in Siberia.

According to the Siberian Times, Vasily Bogoyavlensky called for the investigation when several more of the craters appeared. Back in Nov., researchers went into one in the Yamal Penninsula, but that was one of only a handful at the time.

"We know now of seven craters in the Arctic area," Bogoyavlensky, deputy director of the Moscow-based Oil and Gas Research Institute, told the newspaper. "Five are directly on the Yamal peninsula, one in Yamal Autonomous district, and one is on the north of the Krasnoyarsk region, near the Taimyr peninsula.

"We have exact locations for only four of them. The other three were spotted by reindeer herders. But I am sure that there are more craters on Yamal, we just need to search for them."

When observing the holes from above, the Times noted, they are not confined to one area in particular, but they do seem to be in northern Russia.

"I would compare this with mushrooms: when you find one mushroom, be sure there are few more around," Bogoyavlensky said. "I suppose there could be 20 to 30 craters more."

Once considered a phenomenon brought on by uncharacteristically warm temperatures in the area, the researchers are now concerned.

"These objects need to be studied, but it is rather dangerous for the researchers. We know that there can occur a series of gas emissions over an extended period of time, but we do not know exactly when they might happen," Bogoyavlensky said. "For example, you all remember the magnificent shots of the Yamal crater in winter, made during the latest expedition in November 2014. But do you know that Vladimir Pushkarev, director of the Russian Centre of Arctic Exploration, was the first man in the world who went down the crater of gas emission?

"More than this, it was very risky, because no one could guarantee there would not be new emissions."