Two more craters of unknown origin have been spotted in Russia's Siberia region, weeks after a similar-looking hole was found in the isolated northernmost area, a local paper reported.
The Siberian Times, an English-language newspaper, published pictures of two new giant holes discovered by reindeer herders, one located in the Yamal and the other in the Taymyr peninsula, both above the Arctic circle.
The paper said that theories of their origin range from meteorites or stray missiles to aliens or an underground gas explosion. The report could not be confirmed independently.
Russian state TV reported earlier this month that a giant hole had appeared in the gas-rich Yamal peninsula where temperatures plummet below -50 degrees Celsius and the sun barely rises in winter.
A Russian scientific expedition arrived at the site to inspect the first crater, nicknamed the "Yamal black hole", earlier this month, according to a recent report by state-run Vesti.ru website.
Yamal, inhabited by indigenous reindeer herders, is one of Russia's richest regions in natural gas.
A meteorite, which weighed about 10 metric tonnes, hit central Russia last year, injuring more than 1,000 people.
Experts drew comparisons with an incident in 1908, when a meteorite is thought to have devastated an area of more than 2,000 sq km (1,250 miles) in Siberia, breaking windows as far as 200 kilometres (125 miles) from the point of impact.