mean that in wetter times the lake that once filled Death Valley (Lake Manly) was the last stop for water flowing in the region, meaning the water there was saturated in dissolved materials. Thus, the salt pans in Death Valley are among the largest in the world and are rich in minerals, such as borax and various salts and hydrates. The largest salt pan in the park extends from the Ashford Mill Site to the Salt Creek Hills, covering some of the valley floor. The best known playa in the park is the Racetrack, known for its moving rocks. Climate According to the Köppen climate classification system, Death Valley National Park has a hot desert climate (BWh). The plant hardiness zone at Badwater Basin is 9b with an average annual extreme minimum temperature of 28.0 °F (–2.2 °C). Death Valley is the hottest and driest place in North America due to its lack of surface water and low relief. It is so frequently the hottest spot in the United States that many tabulations of the highest daily temperatures in the country omit Death Valley as a matter of course. On the afternoon of July 10, 1913, the United States Weather Bureau recorded a high temperature of 134 °F (56.7 °C) at Greenland Ranch (now Furnace Creek) in Death Valley. This temperature stands as the highest ambient air temperature ever recorded at the surface of the Earth. (A report of a temperature of 58 °C (136.4 °F) recorded in Libya in 1922 was later determined to be inaccurate.) Daily summer temperatures of or greater are common, as well as below freezing nightly temperatures in the winter. July is the hottest month, with an average high of and an average low of . December is the coldest month, with an average high of and an average low of . The record low is . There are an average of 197.3 days annually with highs of or higher and 146.9 days annually with highs of or higher. Freezing temperatures of or lower occur on an average of 8.6 days annually. Several of the larger Death Valley springs derive their water from a regional aquifer, which extends as far east as southern Nevada and Utah. Much of the water in this aquifer has been there for many thousands of years, since the Pleistocene ice ages, when the climate was cooler and wetter. Today's drier