On a clear day in Gibraltar, looking out of Gorham's Cave, you can see the rugged northern coast of Morocco looming purple above the turquoise sea. Inside the cave, quiet prevails, save for the lapping of waves against its rocky beach. But offshore, the strait separating this southernmost tip of the Iberian Peninsula from the African continent bustles with activity. Fishing vessels troll the waters for tuna and marlin, cruise ships carry tourists gawking at Gibraltar's hulking limestone massif, and tankers ferry crude oil from the Mediterranean to points west. With its swift, nutrient-rich currents, mild climate and gateway location, the area has attracted humans for millennia.