Vladimir Shtanko/Getty Images; Rajesh Jantil-AFP/Getty Images; Maranie Staab/Newscom; Jose del Olmo/Newscom Soon, metal monoliths began appearing and disappearing around the world, from California ...

near where the Tennessee meets the Ohio River. The first completed monolith, called L31, is at the lowest elevation of 50 monoliths to be built under the Corps’ contract with Thalle Construction Co.