Cruise ships are beginning to quietly plug into cleaner power while docked, marking a shift that could significantly cut emissions in port cities. Seattle recently became the first U.S. port where every cruise berth offers shore power connections, allowing ships to turn off their diesel engines and run instead on the city’s largely renewable electric grid. The system wasn’t simple to build: it required a mile-long underwater cable, multiple connection points to fit different ship designs, and careful coordination with local tribes to protect fishing areas.

Miami, the world’s busiest cruise port, has also switched on its own shore power system, the largest in the U.S. Built with Florida Power & Light, it can handle five cruise ships at once, an engineering feat that illustrates how quickly this technology is scaling.