In simplistic terms Assets in the Balance sheet is a summation...

In simplistic terms Assets in the Balance sheet is a summation of all the money that has historically been sunk into building these holes in the ground and ancillary services and plant and equipment that is leased (it is shown as an asset with a corresponding liability). These are historical asset costs and regretably a short fall of accounting principles in this area mean that there tends to be little real relationship to what a person is prepared to pay for those assets today. Market Cap + Debt is effectively the daily market based valuation of those net assets. You will note that some company's directors / auditors ask them to write down their assets by taking a charge to their profit and loss statement to recognise the present day value of those assets. It seems that the Board and auditors of WHC in their wisdom do not think this is required.

In terms of break up value of a coal asset suggest you google / read some issues with trying to sell a coal asset, BHP has reportedly been trying to sell Mount Arthur - one of the biggest thermal coal mines in Australia for quite some time....

Hope this helps.