A canal that delivers vital water supplies from Northern California to Southern California is sinking in places. So are stretches of the San Joaquin River. 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. This slow-motion land subsidence — more than one foot a year in some places — is not expected to stop anytime soon, experts say, nor will the expensive repairs.

“It’s shocking how a huge area is affected, but how little you can tell with your eye,” said James Borchers, a hydro-geologist, who studies subsidence and says careful monitoring is necessary to detect and address sinking before it can do major damage to costly infrastructure such as bridges and pipelines.

Land subsidence is largely the result of pumping water from the ground. As aquifers are depleted, the ground sags. The most severe examples today are in San Joaquin Valley, where the U.S. Geological Survey say half of the land is prone to sinking, calling it one of the “single largest alterations of the land surface attributed to humankind.”

California’s drought has forced farmers in the Central Valley, the nation’s most productive agricultural region, to rely more heavily on groundwater to irrigate their crops. This type of over-pumping has destroyed thousands of well casings and buckled canal linings. At least one bridge in Central California now sits below the waterline. Replacing it is expected to cost $2.5 million. Rebuilding another canal recently cost $4.5 million.

NASA research shows that parts of the California Aqueduct, a massive canal that delivers water 400 miles to Southern California, has subsided by nearly 13 inches. This has cost the state of California “tens of millions of dollars” in repairs in the last 40 years, and officials expect to spend that much in the future, said Ted Thomas of the state’s Department of Water Resources.

Putting a grand total on damage from subsidence in California is tricky because irrigation districts don’t often single out repairs required by subsidence from general upkeep, but Borchers estimates long-term costs as being “probably in the billions.”

Last year, Gov. Jerry Brown signed legislation to regulate groundwater — but local agencies have until 2040 to put groundwater management plans into effect.