He’s referring to subsidence. The buoyant weight of soil is less than the dry weight, so it does compact when the groundwater is drawn down lower than ever before (as it has been for decades, getting lower and lower). It’s an irreversible process.

The California Aqueduct is currently dealing with problems relating to subsidence - what used to be a smooth downhill grade is now slumping in the middle. The capacity of the canal is reduced and without intervention it would eventually be unable to flow uphill.