As the Arctic thaws, it becomes much more accessible—and desirable. An exhaustive 2008 U.S. Geological Survey study determined that 13 percent (90 billion barrels) of the world's undiscovered oil and 30 percent (1,670 trillion cubic feet, or 47 trillion cubic meters) of its undiscovered natural gas lie waiting (map). About half the Arctic Ocean is less than 500 meters deep, readily reachable by drilling rigs where sea ice has retreated. Countries, notably Russia, are building numerous airports, seaports and other infrastructure. And they are expanding military installations to protect assets and sustain increasingly busy shipping lanes.