Robot developers competing for millions of US dollars have sent their technology beneath the surface of Kentucky – taking on the form of troops and first responders.
American forces want to build on systems that can map, search and navigate unstable environments, reducing risk to human life.
Rather than sending individuals into collapsed, darkened and dangerous settings first, they want to send in bots or drones to get a feel of what lies ahead.
Combat and disaster responses in caves, tunnel and urban settings have been replicated by the Defense Advanced Research Projects Agency (DARPA), the US military's research and development unit.
The competition has invited teams to put tracked, wheeled and winged robots to the test since 2018, culminating in the final event for the Subterranean Challenge.
Programme manager Dr Timothy Chung says DARPA "transformed" an American cavern to host the final, which brought all three environments together in a "triathlon".
Systems ran against the clock to report signs of life back to their teams above ground – often relying on autonomy to negotiate dark and unpredictable conditions when outside of signal.
Winners of the main competition, a vehicle team known as 'Cerebrus', took home $2m, while a smaller cash prize was handed out for a virtual competition mirroring the same challenges.
DARPA will take lessons from the event back above ground, building on the best technology to help save lives.