Early 20th-century residents of Fort Bragg, California, disposed of their trash by throwing it over a cliff and onto a nearby beach. Cans, bottles, tableware, and household garbage accumulated in huge, disgusting piles. Even when residents stopped depositing trash on the beach, it remained an embarrassment—a dump seemingly beyond reclamation.

Over the years, however, wave action broke up the glass and pottery and washed the rubbish out to sea. The pounding surf rolled and tumbled the glass fragments in the sand on the ocean floor, frosting and smoothing the surface and creating gemlike “sea glass,” which it then deposited back onto…