Ballard Locks. The Locks were built in Seattle in 1906 to provide a connection for boat traffic between Lake Union and Puget Sound. It's a magical place for kids and adults. Boats come in, the giant water door closes and the water raises or lowers the boats depending on what side of the Lake or Sound boats are traveling. The entire process is open for all to see. Here's a video I took of a sailboat going through the Locks.
I didn't know until a recent visit that the Army Corp of Engineers actually empty the Locks every year for an inspection at which time they get all the dropped sunglasses, cameras and phones (sad!) out of the bottom of the Locks. The date hasn't been announced yet, but they drained the big Locks in November and supposedly the smaller Locks inspection is coming up in March.
Oh and if you'd like to learn more about Hiram Chittenden, the Brigadier General who built the Locks, check this out. He was an interesting guy.