Swarming
In 2014 the U. S. Office of Naval Research released a video showing tests of a swarm of small autonomous drone attack boats that can steer and take coordinated offensive action as a group.

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Myths: There is a popular myth that lemmings commit mass suicide by swarming off cliffs when they migrate. Driven by strong biological urges, some species of lemmings may migrate in large groups when population density becomes too great. Lemmings can swim and may choose to cross a body of water in search of a new habitat. In such cases, many may drown if the body of water is so wide as to stretch their physical capability to the limit. This fact combined with some unexplained fluctuations in the population of Norwegian lemmings gave rise to the myth.
Piranha have a reputation as fearless fish that swarm in ferocious and predatory packs. However, recent research, which started "with the premise that they school as a means of cooperative hunting", discovered that they were in fact rather fearful fish, like other fish, who schooled for protection from their predators, such as cormorants, caimans and dolphins. A researcher described them as "basically like regular fish with large teeth".

See also: Active matter – Matter behavior at system scale
Dyson swarm – Hypothetical megastructure around a starPages displaying short descriptions of redirect targets
List of collective nouns in English
Mobile Bay jubilee – Natural phenomenon that occurs in Mobile Bay, Alabama, United States
Population cycle – the repeated increase and decrease of a species' population over timePages displaying wikidata descriptions as a fallback
Swarm (simulation) – open-source agent-based simulation toolkitPages displaying wikidata descriptions as a fallback
Swirlonic state – recently (2020) recognised new state of self propelled particles
Traffic wave – Type of highway congestion
Swarmalators - Agents that do swarming and synchronization simultaneously.

References: 

Sources: Blum C and Merkle D (2008) Swarm intelligence: introduction and applications Springer. ISBN 978-3-540-74088-9.
Camazine S, Deneubourg JL, Franks NR, Sneyd J, Theraulaz G and Bonabeau E (2003) Self-Organization in Biological Systems Princeton University Press. ISBN 978-0-691-11624-2.
Fisher L (2009) The perfect swarm: the science of complexity in everyday life Basic Books. ISBN 978-0-465-01884-0.
Kennedy JF, Kennedy J, Eberhart RC and Shi Y (2001) Swarm intelligence Morgan Kaufmann. ISBN 978-1-55860-595-4.
Krause, J (2005) Living in Groups Oxford University Press. ISBN 978-0-19-850818-2
Lim CP, Jain LC and Dehuri S (2009) Innovations in Swarm Intelligence Springer. ISBN 978-3-642-04224-9.
Miller, Peter (2010) The Smart Swarm: How understanding flocks, schools, and colonies can make us better at communicating, decision making, and getting things done Penguin, ISBN 978-1-58333-390-7
Nedjah N and Mourelle LdM (2006) Swarm intelligent systems Springer. ISBN 978-3-540-33868-0.
Sumpter, David JT (2010) Collective Animal Behavior Princeton University Press. ISBN 978-0-691-14843-4.
Vicsek A, Zafeiris A (2012). "Collective motion". Physics Reports. 517 (3–4): 71–140. arXiv:1010.5017. Bibcode:2012PhR...517...71V. doi:10.1016/j.physrep.2012.03.004. S2CID 119109873.

External links: New York Times article on investigations into swarming
From the Wolfram Demonstrations Project – requires CDF player (free):
Model of a Firefly Swarm.
Garbage Collection by Ants
Beverton and Merging Schools of Fish
Propp Circles