It is the situation that has changed.

The lake of Tezcuco is four miles off, though the causeways which once connected the city with the dry land still exist, and have even been enlarged.

They look like railway-embankments crossing the low ground, and serve as dykes when there is a flood, a casualty which still often happens.
This change is interesting to the student of physical geography; and Humboldt's account of the causes which have brought it about is full and explicit.

When Mexico had been built a few years, the frightful inundations which threatened its very existence at length awoke the Spaniards to a sense of the mistake that had been made in placing themselves but a few feet above the lowest level of the valley, in such a way that, from whatever point the flood might come, they were sure to get the benefit of it.

The Spanish authorities at home, with their usual sagacity, sent over peremptory orders that the city should be abandoned, and a new capital built at Tacubaya--a proposal something like intimating to the inhabitants of Naples that their position, at the foot of Mount Vesuvius, was most dangerous, and that they must leave it and settle somewhere else.