Ground ice is a collective term used to refer to all types of ice found in permafrost, the layer of ground—often hundreds of feet thick—where temperatures usually remain below zero degrees Celsius. When moisture mixed with soil in this ground freezes, ice patterns known as polygons may appear on the surface, a jigsaw puzzle of stone-rimmed ﬁgures extending across a plain. Called fairy rings in Ireland and patterned ground in parts of the United States, these polygons are generated by expansion—when ice forms, it takes up a volume about eleven percent greater than the volume of water used to create it. Larger solid particles are moved to the outside edge of “bubbles” formed by the freezing process, and stones collect at the junction, forming the fairy rings. Pore ice, which ﬁlls each opening of any available porous material, is one of ﬁve types of ground ice. The others are segregated ice, pingo ice, ice wedges, and ice lenses. The latter is a horizontal accumulation of ground ice in a small area, while segregated ice describes a more extensive area of pure ice that grows because of active migration of water from some nearby source. Ice wedge refers to a vertically oriented wedge-shaped piece of ground ice that forms in the “active” (that is, not permanently frozen) layer of permafrost. And then there’s pingo ice, created by the freeze-thaw cycle in the active permafrost layer. The active layer begins to freeze in winter, ﬁrst at the surface and then at an increasing depth below the surface. At some point this advancing “freeze front” approaches the permanently frozen layer, compressing the remaining free water. Forced upward through narrow openings to the surface, this water raises the upper soil layer in a mound, often with an opening in the middle where any remaining free water comes out (and freezes, of course). These mounds, called pingos, can be found throughout the Mackenzie Delta of northern Canada, though they occur in Alaska, too. The phenomenon is most commonly seen on deltas and other layers of lake sediments in permafrost regions. Scientists searching for proof of past climate change often search for fossil ice wedges—not ice, but a cast in the ground where ice once occurred—and have found them in places such as Pennsylvania, an area of permafrost during the late Pleistocene.