Four years of drought and heavy reliance on pumping of groundwater have made the land sink faster than ever up and down California's Central Valley, requiring repairs to infrastructure that experts say are costing billions of dollars.

Those include a canal that delivers vital water supplies from Northern California to Southern California and stretches of a riverbed undergoing historic restoration. On farms, well casings pop up like mushrooms as the ground around them drops.

This slow-motion land subsidence — more than a foot a year in some places — is not expected to stop anytime soon, experts say, nor will the expensive repairs.

Experts say careful monitoring is necessary to detect and address sinking before it can do major damage to costly infrastructure such as bridges and pipelines.