Authorities can't explain yet why the bottles of water have been left in the tunnels. Most have been found on emergency lights or near emergency exits.
It could be practice runs for bottles filled with "something else" at a later date. - i.e., A study of how easy they are to place? How long before someone noticed?, etc.
A "hoax or prank" of this type should result in jail time. This is not a day and age when this kind of thing can be taken lightly. We are at war and the enemy is on our shore. I think Tom Ridge should make a public statement to this effect.