Derweze was a town in Turkmenistan until it was destroyed.
Read more about the history of the town from Wikipedia.
Gas exploration in the area has started, therefore there is now a sandy track that goes from the main road to the crater.
The turnoff is around 1,5km South from the point where the railways cross the main-road. Coming from the North, you must get-off at the top of a small climb where 2 side roads in every direction are facing each other.
Turn left on the sandy track and follow it for about 7 kms. There is a gas well (under construction in april 2012) after 5 kms and many trucks travel all day long between it and the main road, and getting a lift is easy. To the crater itself, follow the road for another 2 kms.
Do not forget to stock up water. If coming from North, there are some chaikanas 1km before the railways line where you can buy water.
Getting a ride further is fairly easy, traffic is good on this road with many trucks in both directions, so do not hesitate to give up a lift to explore the area.