It is thought that the United States military has a small fleet of black triangular airships in current service for cargo lift purposes. These airships could stay up for weeks at a time at high altitudes traversing the planet from point to point.
Airships historically have held the record for both payload capacity and altitude. The altitude record is still held by lighter-than-air (LTA) vehicles except for research craft such as the X-15 and the United States space shuttle.
The military would have use for these type of aircraft for several different purposes. One of course is for heavy cargo transport. You could have a cargo bay in the bottom of the airship and drive armored vehicles up into it, go to the combat area and offload them, quickly, and safely. Another possible role is that of a support gunship, similar to the AC-130 Specter, for bombardment of enemy ground targets.
Also in recent military history the use of Forward Arming and Refueling Points (FARP) has become essential to fast and effective helicopter combat. An airship could act as a mobile FARP, eliminating set up time and the allocation of personell to run such a facility, as well as give the support crews the ability to pack up and retreat or advance at any given moment, as well as greatly improve helicopter tactics allowing for greater support from dedicated gunships.
A possible propulsion system for these airships could be an electrokinetic field drive (powered by a nuclear device) and would use no propeller or jet type drive system. This would mean that the airships would be almost completely silent except for an electric humming from the power units. These could be supplemented by solar cells, fuel cells, and other power systems.
Of course there are some people who believe that these airships (If they are that) are really spacecraft from extraterrestrial beings come to visit our planet. I think it more probable (As many of you might agree) that these are airships operated by the military for transport and support purposes.
I would like to note that most of this is speculation as to what these objects might be and not what they are.
Leonard David, Investigation Casts Light on the Mysterious Flying Black Triangle, http://www.space.com/businesstechnology/technology/black_triangle_020805.html, August 9, 2002