The Door To Hell

In the desert of Turkmenistan is a massive crater which has literally been on fire for decades.

During the 1970s, when Turkmenistan was still part of the USSR, geologists were searching for gas in the desert - often detected as seeping through the sand. While drilling, the geologists opened up a massive hole full of poisonous gas. The hole, ~100 meters in diameter, easily swallowed their equipment. To burn off the excess poisonous gas, the geologists set it ablaze thinking it would die out in a few days time.

Fail.

Here's a video of the Darvaza Gas Crater on fire (starts at :40).