Water infiltration among gravels on Rialto Beach, Washington 

Here's a cool phenomenon I noticed on Rialto Beach on Washington's Olympic Peninsula. It involves infiltration of water into the gravels on the beach as waves recede. About 42 seconds into this video, you can see the phenomenon. A wave recedes and as the water infiltrates, patches of the gravels will suddenly appear darker. It's a cool "special effect," brought to you by nature.