This is a bizarre, perhaps slightly surreal spectacle – the ant vortex, also known as an ant mill. Apparently, according to Wikipedia,
An ant mill is a phenomenon where a group of army ants separated from the main foraging party lose the pheromone track and begin to follow one another, forming a continuously rotating circle. The ants will eventually die of exhaustion. This has been reproduced in laboratories and the behaviour has also been produced in ant colony simulations. This phenomenon is a side effect of the self-organizing structure of ant colonies. Each ant follows the ant in front of it, and this will work until something goes wrong and an ant mill forms. An ant mill was first described by William Beebe who observed a mill 1,200 feet (365 m) in circumference. It took each ant 2.5 hours to make one revolution. Similar phenomena have been noted in processionary caterpillars and fish.
Check out footage below:
