A very important feature of the more humid permafrost areas is the existence of permafrost mounds, e.g., the well-known pingos (Figure 7.1), although similar mounds have been reported from the drier locations and locations where springs may develop in otherwise arid regions, e.g., Mongolia (Skyles & Vanchig, 2007), and the Qinghai-Tibet Plateau (see Figure 1.19). They range in size from over 250m in diameter to small hummocks less than a metre across. Some have steep slopes while others have low slope angles. Most are permanent features of the landscape under stable climatic conditions, whereas others are either unstable or seasonal. Some appear in a single winter while others develop over a period of one or more centuries. The substrate may be peat, mineral soil, or both.