IT doesn't matter if it is coal or potash, mining is a harsh and unforgiving environment.

But the mine at Boulby, at the very edge of the North York Moors, takes things to a different level entirely.

About a mile below ground, buried under thousands of tonnes of rocks and millions of years of geology, lies a rich seam of one of the purest forms of one of the world's most valuable and sought-after minerals - polyhalite.

Although attractive in its own right - the opaque crystal varies in colour from clear to grey and can be polished up to be set into jewellery - it has a far more important and commercially-viable use.

The mineral is highly prized because it can be processed into potash - an essential component of fertiliser.

  • Scientists are using the mine to probe whether life can exist on Mars. Boulby was chosen because its dark, salty environment is similar to that on planets in outer space.
  • The Boulby Underground Laboratory, the UK's deep underground science facility, is located 1100m below ground.
  • Studies underway at Boulby range from the search for Dark Matter in the Universe, to studies of cosmic rays and climate, astrobiology and life in extreme environments, development of techniques for deep 3D geological monitoring and studies of radioactivity in the environment.
  • Potash and salt have been extracted from the ground there since the 1970s, but the rocks also hold microbes, too small to see by eye, called extremophiles.
  • Cleveland Potash has been East Cleveland's biggest business and biggest employer for many years.
  • The Boulby mine has been producing the potassium-rich salt mineral since 1973. Used almost exclusively in the production of agricultural fertilisers, about one million tons of potash is extracted each year, half the UK's total output. The mine also produces rock salt for use on roads in winter.
  • In 2010, the mine stepped up road salt production in a bid to help keep the nation moving, during the worst winter in a generation.
  • The mile-long hole in the ground was excavated in 1969 when work started on Boulby mine, which sits nestled on the edge of the North York Moors overlooking the North Sea.
  • The enormous grassed mounds looming over the mine's railway are testament to the amount of earth removed to sink the shaft. The railway itself used to transport most of the mined material to Teesport, where it is shipped around the UK and further afield.
  •  The deeper you go, the hotter it gets. The temperature at the face is about 42 degrees.
  • Workers get around in the Pink Panther, a bright pink Land Rover that has been converted for use underground.
  • The tunnels themselves are enormous, easily large enough for two vans to pass side by side and with a height of about 15ft, far from the claustrophobic burrows miners endured in the heyday of North-East coal mining.