It can be useful to think about data as a bit like petroleum. Oil deposits exist along a continuum of extraction difficulty.[136] Some oil gushes out of the ground under its own pressure, ready to refine and cheap to produce. Other deposits need expensive deep drilling, hydraulic fracturing, or special heating processes to extract it from shale rock. When oil prices are low, energy companies extract oil only from the cheap and easy sources, but as prices rise it becomes economically viable to exploit the tougher-to-access deposits.
