Look carefully at this aerial view and you can see lines of darker and lighter vegetation slicing across the crop rows. This week's issue of Science magazine features an article by Paolo Mozzi, an Italian geomorphologist who surveyed this same field with infrared cameras during the severe drought of 2007 to analyze subtle variations in plant hydration.
He was looking for crop marks, an archaeological technique that looks at variations in plant health on the surface as a way to discover ancient walls and building foundations that have since been buried beneath three feet of soil. At right is a sketch from Wikipedia that shows how it works: a stone wall just below the surface (on the left side of this sketch) limits the amount of topsoil and stunts the growth of the plants growing above, while an old ditch (on the right) helps the plants above it grow taller with extra topsoil.
Here's what Mozzi's infrared survey found:
This is the ancient Roman city of Altinum, a place that became an important strategic and commercial center thanks to its position at the edge of the Venetian lagoon and at the crossroads of the empire's Via Annia and Via Claudia Augusta. In the fifth century C.E., it and several other Roman cities nearby were conquered and burned by Attilla the Hun, and subsequent barbarian invasions from the north forced the Romans to retreat from the mainland cities to the marshy islands of the lagoon.
Altinum was abandoned completely by the 11th century, but its island refugee settlements grew in size and prosperity to eventually become the city of Venice.
The Roman historian Strabo wrote that "Altinum too is in a marsh... and hence subject to inundations"(see note below). The engineering behind the canals of Venice was pioneered in Altinum, where the Romans built a network of canals to drain the marshes and carry the water away from their buildings. One of the canals is clearly visible in the infrared image above, and, if you know where to look, it's also visible as a slightly darker band of green in the aerial view at the top of this post. When the Romans left, the canals filled in and the marshes took over once again, until the nineteenth century, when the land was reclaimed for farms.
This makes Altinum one of the few Roman cities that wasn't subsequently buried underneath more modern settlements. Instead, it sank into the marshes, only to be re-discovered a thousand years later as a shadow cast over an unwitting farmer's agricultural yield.
Note: Here's the translated source text from "The Geography of Strabo." Strabo calls Altinum similar to the nearby city of Ravenna, which sounds even more like modern Venice: he calls it "a city built entirely of wood [another possible translation is "built on piles"] and coursed by rivers, and it is provided with thoroughfares by means of bridges and ferries. At the tides the city receives no small portion of the sea, so that, since the filth is all washed out by these as well as the rivers, the city is relieved of foul air."