The constant underground temperature of 12 °C (54 °F) has been utilized in preserving ice collected
during winter months in the form of underground pits since Roman times.
The temperature of the soil is held relatively constant year-round when taken below the frost line,
located 0.9 to 1.5 m (3 to 5 ft) below the surface,
and varies from about 7 and 21 °C (45 and 70 °F) depending on the region.
Prior to the convenience of having refrigeration inside the home,
cold storage systems would often be located underground in the form of a pit.
These pits would be deep enough to provide thorough insulation
and also to deter animals from intruding on the perishable items within.
Early examples used straw and sawdust compacted along the sides of ice to provide
further insulation and to slow the ice melting process.
By the year 1781, personal ice pits were becoming more advanced.
The Robert Morris Ice House, located in Philadelphia,
brought new refrigeration technicalities to the forefront.
This pit contained a drainage system for water runoff as well as the use of brick and mortar for its insulation.
The octagon-shaped pit,
approximately 4 meters in diameter located 5.5 meters underground
was capable of storing ice that was obtained during the winter months to the next October or November.
Ice blocks collected during winter months could later be distributed to customers.
As the icebox began to make its way into home during the early to mid 19th century,
ice collection and distribution expanded and soon became a global industry.
During the latter half of the 19th century,
natural ice became the most second most important US export by value, after cotton.
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