It's the largest cooperative unit ever recorded, according to Swiss, French and Danish scientists, whose findings appear in Tuesday's issue of Proceedings of the National Academy of Science.
The colony consists of billions of Argentine ants living in millions of nests that cooperate with one another.
Normally, ants from different nests fight. But the researchers concluded that ants in the super-colony were all close enough genetically to recognize one another, despite being from different nests with different queens.
Cooperating allows the colonies to develop at much higher densities than normally would occur, eliminating some 90 percent of other types of ants that live near them, said Laurent Keller of the University of Lausanne, Switzerland.
The Argentine ants were accidentally introduced to Europe around 1920, probably in ships carrying plants, Keller said in an interview via electronic mail.
Richard D. Fell, an entomologist at Virginia Polytechnic Institute, said Argentine ants have been known to form large colonies - the size of several city blocks, for example - but he had not heard of any as large as that cited in the new report.
"It may be that certain ant colonies will bud off, form satellites and remain connected with one main colony," he suggested.
The European researchers said that in addition to the main super-colony of ants they found a second, smaller but also large colony of Argentine ants in Spain's Catalonia region.
When ants of the two super-colonies were placed together they invariably fought to the death, while ants from different nests of the same super-colony showed no aggression to one another.
"It is interesting to see that introduction in a new habitat can change social organization," Keller said of the behavior of Argentine ants that had been relocated to Europe. "In this case, this leads to the greatest cooperative unit ever discovered."
However, in the long run the very cooperation that seems to make them successful could lead to the super-colony's self-destruction, he suggested.
That's because in such a giant colony many workers are unrelated to the queens they help to raise. "Thus, in the long term, selection should decrease the altruistic behavior of workers," he said, because their efforts are not helping transmit copies of their genes via related queens.