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Polygonal ground (or polygonal patterned ground) is a kind of patterned ground, i.e., ground with well-defined natural patterns. Polygonal ground has patterns made up of straight lines joining to form interlocking polygons. Such patterns occur on Earth, in some ground near the poles (e.g., northern Alaska), and is credited to sublimation of ice within the ground, and considered evidence of permafrost and that water exists in some form within the ground. Such patterns have been observed on Mars, suggesting similar conditions at some time. Formation has been simulated and models have been developed that relate the polygon size to the conditions that formed it.