Federal and state government agencies and BP have entered a new and important phase in determining how much damage has been done to the ecosystem by the Gulf of Mexico oil spill.

On Wednesday, the government and BP said they had started to determine how much money it will take to repair the ecosystem damage.

This is part of a legal process known as a natural resources damage assessment, or NRDA (pronounced Ner-da). A damage assessment is done after an oil driller causes damage to a public resource.

In the three months before a temporary cap stemmed the flow from BP's blown-out well, as much as 172 million gallons of oil and millions of cubic feet of natural gas spewed into Gulf waters.