This salt is formed by the evaporation of sea water of a long period of time and often in multiple layers. As it forms minerals  such as potasium, iron and gypsum are absorbed into the halite causing it to change color and flavor.  

A subset of these deposits are referred to as salt domes. When geological pressure forces a section of salt up through the rocks  salt domes can form. While these domes are often measure approximately a mile or so across they tend to be much nearer the surface reducing the difficulties of deep mining. 

Historically Significant Halite Mines include the Khewra Salt Mine  in Pakistan allegedly discovered by Alexander the great,  Wieliczka Salt Mine in Poland which features underground chapels and statues carved from salt, and the Hallstatt Salt Mine in Austria with Bronze Age origins (around 1450 BC), its control shaped the influential Hallstatt culture of the Iron Age and was a cornerstone of the Habsburg empire.