CNN News:北极地区出现神秘陨石坑

Where would you find the largest known impact crater on Earth? South Africa, Siberia, Antarctica, or Mexico. South Africa's Vredefort crater is the world's largest known impact crater.
An impact crater, as the name suggests, is formed when something like an asteroid crashes into something else like Earth. This more recent phenomenon was not caused by an impact but it is one of 17 craters that have mysteriously appeared in the Russian arctic since 2013.
Scientists don't know for sure how this happened. They do know the hole's about 150 deep. Picture three city buses measured end to end and after it was discovered last summer, they flew a drone inside the crater to capture dozens of images from which they built a three-dimensional model. That helped researchers to hypothesis this.

Methane gas somehow accumulates in the permafrost of Siberia. They're not sure whether the gas comes from deep inside the Earth or near its surface but they believe that as the methane builds in a cavity in the ice it forms a mound. And that mound grows until if finally explodes, blasting ice and other debris and leaving behind the crater.
Scientists are using computer technology to try to predict where this will happen next. These craters appear in sparsely populated areas. They're usually discovered on accident by reindeer herders or during helicopter flights of the area.