The Aztec capital, Tenochtitlan, was built on an island in Lake Texcoco. The city was constructed on what was originally a small swampy island, which the Aztecs expanded using a system of artificial islands and chinampas (floating gardens) for agriculture. This location was chosen after the Aztec prophecy saw an eagle with a snake in its beak perched atop a cactus on the island. Tenochtitlan became a large and powerful city-state centered on this lake island in the Valley of Mexico.

Thus, the blank can be filled with: "an island in Lake Texcoco."