From Zelda Wiki, the Zelda encyclopedia
A Warp Jar takes the appearance of a bubbling cauldron, with fumes of one of three different colors. There are three jars in each dungeon, usually beginning with a green one near the entrance of the dungeon, a blue one in the middle, and yellow one next to the boss's door. If Link climbs into the first one, he will be transported to the location of the second one, and if he climbs into the second, he will be transported to the third, providing the jar has been found and 'opened'. Climbing into the third Jar transports Link to the location of the first one, providing an easy exit, should one be required. Jumping into a jar while none are opened will merely cause Link to jump out and stay in the same room.
Warp jars must sometimes be activated by removing their tops. This may involve using fire to burn off a wooden lid with a Boko Stick or Fire Arrow, or by blowing up a rock using a Bomb. For whatever reason, a whistling sound can be heard in rooms with Warp Jars.