Deep salt

I happened to be watching Michael Portillo wandering around England on trains again on SBS last night, and was surprised to learn about an extensive underground rock salt mine that still operates at Winsford.  Here's a BBC article about it, as well as the mine's own website.

Started in the 19th century, it's about 150 m underground and huge - 160 miles of tunnels, and vast open spaces supported by pillars of salt left in place.  (I'm a bit puzzled how they know, structurally, how wide a space they can leave unsupported):

A part of it is now also used to store archive materials!

This rock salt was all laid down 220 million years ago, when England had salty inland seas.  

This is something I found completely unexpected and very interesting...