Byzantine Coin, Sinop, Turkey, 2001Photograph by Randy OlsonExplorers pulled this ancient Byzantine coin from a Black Sea wreck near Sinop, Turkey. Millennia ago, the Black Sea was a freshwater lake. When the last ice age waned some 12,000 years ago, salt water from the Mediterranean breached the Bosporus Valley, transforming the Black Sea into its current state—a toxic, brackish, oxygen-deficient pool, perfect for preserving ancient wrecks.