The mud volcanoes are listed alongside such sites as the Grand Canyon, Mount Fujiyama and Poland's Masurian lakes.
Online and telephone voting for the top seven is under way and will continue until the middle of 2011.
The New7Wonders of Nature campaign started in 2007, immediately after the successful campaign to elect the man-made Official New 7 Wonders of the World, in which more than 100 million votes were cast.
The New7Wonders of Nature is now in its third and final phase in which the wonders themselves will be chosen by an estimated one billion votes. The shortlist was compiled through national qualification and a panel of experts recommendation from over 440 participants representing over 220 countries.
Mud volcanoes are a sign of the presence of oil and gas underground. They are created when mud and sand up to several kilometres beneath the Earth's surface are pushed upwards by compressive forces and expelled. Azerbaijan has the highest concentration of mud volcanoes in the world.