Only a few years ago, the clear, shallow waters of Mar Menor, a saltwater lagoon off eastern Spain that is Europe’s largest, hosted a robust population of the highly endangered fan mussel, a meter-long bivalve. But in 2016, a massive algal bloom, fueled by fertilizer washing off farm fields, sucked up the lagoon’s oxygen and killed 98% of the bivalves, along with seahorses, crabs, and other marine life. The suffocating blooms struck again and again, and millions of dead fish washed onto shore. By last year, local residents—some of whom benefit from tourism to the lagoon—had had enough. Led by a philosophy professor, activists launched a petition to adopt a new and radical legal strategy: granting the 135-square-kilometer lagoon the rights of personhood. Nearly 640,000 Spanish citizens signed it, and on 21 September, Spain’s Senate approved a bill enshrining the lagoon’s new rights.