With its bright blue waters and dramatic cliffside villas, Santorini, Greece, is an idyllic vacation destination, but lurking beneath the Aegean waves lay the remains of one of the most active volcanoes in human history. Because of its eruptive past and proximity to population centers, the Santorini Caldera is closely monitored, but a recent expedition revealed something never before seen there or anywhere else: shimmering opalescent pools of carbon dioxide sequestered on the seafloor.