Is an old Aztec legend.
Octacorpus might know it better than me but, here it goes: someone told the Aztecs to build their capital wherever they find an eagle eating a snake, on top of a cactus.
The Aztecs travelled and searched far and wide until they found it in the middle of a lake (the Lake Tenochtitlan), and on that same place is where they founded their capital, Tenochtitlan, today called Mexico DF.