One of Northern Ireland's great natural resources is the Rock Salt mine under the Belfast Lough at Carrickfergus
The Irish Salt Mines at Carrickfergus on the edge of the Belfast Lough is the only salt mine in all of Ireland.
It has been in use since 1965 with the Irish Salt Mining and Exploration company extracting as much as 30,000 tonnes of salt per week for use all over Ireland, the UK and as far away as the USA.
The salt is part of a salt bed stretching all the way to Russia with the caves formed from the ongoing mining running for 30 miles up to 1200ft underground.
The salt bed was formed during the Triassic period 220million years ago when a saltwater sea was dried out under the sun to leave a salt desert. It was discovered by miners in the 1800s who were looking for coal deposits.
The rock salt is produced by room and pillar dry milling. The salt bed is undercut, drilled then blasted.
The salt is then crushed and screened at a plant underground and then transported using a conveyor to the facilities at the surface up to 2km away where it is treated with an anti-caking agent (to prevent it clumping should it get damp) and then stored for despatch.
The salt mined at the Irish Salt mines in Carrickfergus is very different to salt that you would use in a domestic setting.
It is coarse and brown, resembling gravel more than salt.
The salt has a grit mix which comes from the siltstone and sandstone from which the rock salt is blasted. This grit is know as Marl.
It is not added to the rock salt grit but does have the convenient effect of providing extra grip under your feet and under the wheels or vehicles.
BBC : Pure Grit: The Carrickfergus mine from the Triassic period
BBC : Seven day week for salt miners
BBC : Carrickfergus salt mine ready for another busy winter
Wikipedia : Salt Mining