Please answer a question about the following article about Mexico City:

The Historic center of Mexico City (Centro Histórico) and the "floating gardens" of Xochimilco in the southern borough have been declared World Heritage Sites by UNESCO. Famous landmarks in the Historic Center include the Plaza de la Constitución (Zócalo), the main central square with its epoch-contrasting Spanish-era Metropolitan Cathedral and National Palace, ancient Aztec temple ruins Templo Mayor ("Major Temple") and modern structures, all within a few steps of one another. (The Templo Mayor was discovered in 1978 while workers were digging to place underground electric cables).

Who designated the floating gardens as a World Heritage site?
UNESCO