If you see a big bridge or stadium suddenly go dark tonight, don't be alarmed.
Seattle and Tacoma are joining thousands of cities around the world and turning out the lights forEarth Hour, from 7:30 to 8:30 p.m.
Landmark buildings will go dark for the hour, and residents are encouraged to take part by turning out all non-essential lights to support the ongoing fight against climate change.
City Hall, Key Arena, Pacific Science Center and Hammering Man are among the facilities turning out the lights in Seattle. The Tacoma Dome, Convention Center, Murray Morgan Bridge and Cheney Stadium are taking part as well.
Earth Hour began in Sydney, Australia in 2007 and has been gaining momentum even since.
This year, 157 countries are taking part in the annual lights-out event.