For decades, a costly problem has been worsening beneath California’s San Joaquin Valley: the land has been sinking, driven by the chronic overpumping of groundwater. As agricultural wells have drained water from aquifers, underground clay layers have compacted and the ground surface has been sinking as much as 1 foot per year in some areas. New research now shows that large portions of the San Joaquin Valley have sunk at a record pace since 2006. “Never before has it been so rapid for such a long period of time,” said Matthew Lees, the study’s lead author.