There’s a time bomb ticking in California’s Sacramento-San Joaquin Delta. The largest estuary on the West Coast of the Americas, the Delta is a network of some 70 islands protected by more than 1,000 miles of levees. The soil on these islands is some of the richest farmland in the world because it is composed of organic material: decaying plants that accumulated over millennia. But when the levees were built 150 years ago to create farms, this dried out the soil, causing it to oxidize and decompose.