 While modern drilling technology makes it possible to drill wells in up to 3 km of water to verify the exact composition of the geology, one half of the ocean is deeper than 3 km, leaving about a third of the planet beyond the reach of detailed analysis.In addition to uncertainty in real reserves, there is significant uncertainty in technological and economical factors that impact what percentage of reserves can be recovered gainfully.In general the easiest to reach deposits are the first extracted.