An unusual event to witness, the death spiral (a.k.a. circular milling) occurs when ants get separated from the colony and start following the pheromone trail of other ants in a circular pattern. The ants will continue the death spiral until they die from exhaustion (unless a lucky few manage to escape).
Ants Circle [via Geeks are Sexy]
Here is a second video of this phenomenon that we found for you.
dancing ants [YouTube]
Want to know more? See this interesting bit of information about a 1,200 ft. circumference ant death spiral observed in 1921.
Ant Death Spiral [The Ant Room]