The Pacific Sand Crab (Emerita analoga) is one of the delightful creatures you can find in the surf of beaches along the Pacific Coast of North and South America. These small (up to 1.5 inches / 3.8 cm) crustaceans are a delight for kids and adults to find and observe. As waves lap the beach, the crabs will swim into the water, and then burrow back into the sand as the water recedes. Here is a video showing how the sand crab can bury itself in less than two seconds:
I posted this sand crab video on twitter, and received an interesting response from biologist Zen Faulkes. He published several papers on his experiments investigating the neurobiology and mechanics of how sand crabs dig (1997a, 1997b, 1998).