Tunnel and cave flying UAVs?

Reported here, "US defence research agency wants to develop air vehicle small enough to fly inside caves and tunnels."

“The warfighter needs something that can navigate inside a building, cave or tunnel, where MAVs can’t go,” says NAV programme manager Darryl Pines. “We need something even smaller that can fly through a window and provide situational awareness inside a building.”
DARPA’s goal is to “provide a clear direction for development of a system that will be useful to the warfighter,” says Pines. “We understand the fundamental physics. This is about integrating technology into a vehicle that can perform a notional mission.” The requirement is for an air vehicle that can fly more than 1,000m (3,280ft) at 7-10m/s, loiter at 0.5m/s for more than 60s then hover in place for another minute or more, sending video back to the operator. The vehicle could be expendable or reusable, he says.

Gosh, I wonder why tunnels and caves are mentioned?

Hat tip to: UAV Blog

Photo is of a micro UAV or "MAV" from here, but it seems that, as with the IPOD, technology marches on.