In summer 2011, residents of Pingquan County in Northern China were surprised to stumble upon frigid ground when hiking up a gentle mountain slope. Despite a meager elevation of just 900 meters and hot air temperatures exceeding eighty degrees, the earth was frozen solid! Their bewildering discovery quickly caught the attention of researchers at the Chinese Academy of Sciences, who set out to study the climate-defying soil. They just reported their assessment of the site in the journal Scientific Reports after four years worth of observations.

What they found was a tidy collection of permafrost just beneath the top soil, roughly eighty meters long, twenty meters wide, and ten meters tall (see figure below). Permafrost, ground that remains at or below zero degrees Celsius for at least two consecutive years, is quite common, underlying roughly a quarter of Earth's surface. But in Pingquan County, where summer temperatures can climb into the nineties, permafrost really shouldn't exist.

Sixteen other anomalous areas of low ground temperature have been identified on Earth, but what sets the newfound cold spot (L-15 in the figure below) apart from the rest is its location. The site is more than 600 kilometers south of the southernmost limit of permafrost on the Eurasian Continent, the researchers say. The only other anomalous cold earth in the Northern hemisphere that's farther south rests atop the Qinghai-Tibet Plateau at an elevation of 4,700 meters.

The researchers attribute the permafrost's existence to the site's unique composition. A course, blocky layer of soil is sandwiched between a layer of clay below and an insulating layer of peaty soil above.

When air temperature drops sharply in mid-November, the cold, dense air penetrates rapidly into the voids between the blocks, resulting in a near-heterogeneous temperature distribution in the blocky layer from top to bottom due to air convection within the porous material. During the early thaw period... there is no convective heat transfer between the warmer (lighter) air above and the colder (denser) air below. Such a thermal regime indicates the heat exchange between the coarse blocky layer and the ambient environment is relatively instantaneous and intense in winter, while prolonged and feeble in summer.

Source: Niu, F. et al. A naturally-occurring ‘cold earth’ spot in Northern China. Sci. Rep. 6, 34184; doi: 10.1038/srep34184 (2016)