"Black triangles" is the name given to a class of unidentified flying objects (UFOs) with certain common features which have reportedly been observed from the 1940s (and possibly earlier) to the present. They have appeared most commonly over cities of the United States and England, but have been spotted worldwide, including a mass sighting over St. Petersburg, Russia, on February 19, 1997.
Since that event, hundreds of observers have reported enormous, totally silent, black triangular craft hovering or slowly cruising at low altitudes over cities and highways, usually at night and making no attempt to evade detection. The craft are even described by many observers as having some sort of "running lights", either bright white lights or pulsing colored lights. These lights usually appear at each corner of the triangle. Sometimes a red pulsating light is seen at the center.
These objects are also sometimes known by the names "big black deltas" (BBDs) and "flying triangles". No conclusive explanation has been found for black triangles. Accordingly, their origin and existence remain shrouded in mystery. - wiki
See my UFO article for a nice picture of what seems to be a military black triangle parked on the ground.