Your favourite unsolved mystery?

The Philadelphia Experiment was just testing low observability using counterlighting to hide shadows that would make a ship contract with background light levels.

http://www.navweaps.com/index_tech/tech-058.php

The ship was at sea in normal conditions and being tracked visuallyand by radar. When the counterlighting floodlights were turned on,those ships that were positioned so the target ship was silhouetted againstthe bright horizon saw it "apparently" vanish to re-appear when the lightswere switched off. Since the ship was under way at the time, thismade it look as if she had moved from one spot to another. Its importantto note that at no time did the aircraft tracking the ship visually, shipstracking her by radar or those ships not in the correct aspect to her losecontact with her. The experiments showed that counterlighting workedbut only in comparatively narrow conditions and it wasn't really a practicaltechnique. It was more or less abandoned (although I have heard thatit may have been used by some merchant ships). Note that some ofthe effects ascribed to the "Philadelphia Experiment" (which was actuallyheld in the West Indies and did not involve the Eldridge) are exactly thoseone would expect to get from staring straight at a brilliant light.