Query: How was Templo Mayor discovered? 
Context: 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).