This massive crater -- Batagaika crater, or what local people call the Gateway to the Underworld -- appeared in Siberia 25 years ago after trees were cut in the 1980s or early 1990s. (Cutting down vegetation reduces insulation of the soil, leading to permafrost melting.) This crater is a kilometer long and 90 meters deep, and is widening by up to 20 m/yr. Such crater are called "megaslumps."
The Independent has another amazing picture of a geologist standing at the base of the cliffs.