My friend Mike Frizzell sent me this very cool story of a very weird playground in Argentina.
Argentina - Playground Swing Moves Mysteriously 12-13-07
EiTB (Euskadi, Spain)
The "paranormal" swing movement drives Argentinians mad
It is not an advertising strategy or a magic trick. This town's fame started in June this year, when one of the three swings in Belgrano Square started moving by itself, while the rest remained still.
The mere movement of a swing has made Firmat, a 25,000 inhabitant town, be internationally known, on the northwest of Buenos Aires (Argentina), the Spanish 20 minutos reported on its Thursday's web edition (text in Spanish) [Video link below - M.A.F.].
This town's fame started in June this year, when one of the three swings in Belgrano Square started moving by itself, while the rest remained still. Gradually and due to the continuous rumors, the paranormal swing's case caught national newspapers' interest and later on, it reached the international ones.
The swing was robbed after a while to be sold in Internet and was replaced with another one. Even if they thought it would stop moving, it did not, but the other two started.
No magic tricks moved the swing and too many people had seen it happen live to think that it was an advertising strategy.
The answer to the enigma [...perhaps - Ig]
Physicists give a possible but complex answer to this phenomenon: resonance. Swings, as all mechanic systems, can have resonance problems that can be influenced by the environment and position. Wind acts as an external force that stimulates the mechanical system in that frequency and drives it. As each swing has a different frequency, one of them could move but not the rest.
(A video of the phenomenon can be seen at link below –M.A.F 12-14-07)
and after swingset replacement