I think there are a couple in the world, but I haven't heard of any in australia. And DEFINETELY none for coal, as it just wouldnt be profitable removing that much overburden unless the seam-depth was like.. 50m+...
Just to clarify they start out with the spiral shape downwards, then they move accross the mineralisation by dumping burden onto one side (high wall) while digging out the otherside (low wall)..
this allows them to rehabilitate as they move.
Did a quick google for you endless, the _DEEPEST_ mine in the world is 1200m deep (0.75 miles)
http://www.losapos.com/openpitmines
it operates for copper, gold, silver, molybdenum and platinum group metals at decent grades.
