Russian research team climbed into the 35-metre hole which was discovered in July this year in the remote northern Russia.
The team attempted to discover the origins of the crater which appeared after an unexplained eruption that flung soil and rock 120 meters from the site.
It was initially thought it might be related to fossil fuel exploration, as it is 25 miles from the largest gas field in the region, but after a radar scan of the ground the team said it was most likely to be due to natural causes that would require further research.
The hole was first filmed in July from the helicopter. However, the researchers had to wait for the sides and water to freeze before they could enter it.
There are numerous speculations about the causes, from shale gas explosions to meteorite strikes.