SENDAI, Japan—It’s a blustery day and I’m standing on a giant concrete seawall, staring over the water.

The seawall, as you might imagine, is a tall, sloped structure intended to keep the sea and the land somewhat apart. It stops the waves from beating down on the shore, whether during a gales of a tsunami or the daily lap of erosion. But the wall itself holds less interest to me than its supporting cast members a few hundred yards out into the water.

A stone’s throw off the coastline, dozens of gigantic, 10-ton concrete structures known as Tetrapods form a long row and look—if we’re being honest—as if a giant dumped a behemoth box of jacks into the sea. It is simultaneously majestic and odd.