The holes, on Russia's Yamal Pensinsula, captured attention after one was first spotted by an aircraft pilot in July, who took this pic:
The world went mad. Suspected causes ranged from meteorites to underground explosions to extra-terrestrial.
Now a team from the Russian Centre of Arctic Exploration has climbed down to the bottom of the largest hole, about 16m, to stand on a frozen lake which itself is about another 10.5m deep.
The team had to brave winter temperatures of -11C, as the crater was a mudbath in summer with water pouring into it from the ground above.
Vladimir Pushkarev, director of the Russian Centre of Arctic Exploration, took a few amazing snaps of the expedition.