Gates of Justice

The Gates of Justice (正義の門, Seigi no Mon) are three massive structures that are each located at the Marine Headquarters, Impel Down, and Enies Lobby. They resemble large gates built into the air with the symbol of the World Government on them. They are built in a triangle formation that forms a water current called the Tarai current, inside the triangle.

Usage
The purpose of these gates is to allow easy travel for Marines between the three respective places they are located in. Beyond the gates is a massive spiral water current that is in between the three locations. By riding this current, the Marines are able to travel much faster to these locations than they would normally. Traveling normally through on this current while the gates are closed however would simply force any ship to pass on by the locations due to the current. When the gates are open, the current is let in and any ship can enter their destinations with ease.

The current that flows inside the three Gates of Justice is called the Tarai current. If criminals somehow managed to enter it, they would most likely be trapped. The reasons for the criminals being trapped is that the World Government would refuse to open the gates again for the criminals and they have no way of escaping the Tarai currents.

Enies Lobby Gate
The gate here in Enies Lobby is located at the very back of the judiciary island and is considered the final location. Once you pass beyond the gate, the only two places you can go are Marineford or Impel Down. Once a criminal passes through the gates, there is no longer any hope left for them, until Luffy escaped with 240 other prisoners.

When the gate is closed, massive whirlpools are formed behind the Tower of Justice due to the current of the waterfalls surrounding the island. This creates an area that no ship can maneuver. For regular standard sized ships to pass through, the gate only need be opened a little bit. However since the whirlpools will continue to turn even if the gate only opens just a little bit, travel is still inaccessible. To solve this, an underwater tunnel was built that goes from the Tower of Justice, under the whirlpools, to a structure located in the middle of the gate, the Bridge of Hesitation. Once the gate opened up a little bit and the bridge is up, one can go to a pier located at the other side and ride the current.

However in order to safely go directly to and from the island, the gates must be fully opened so the whirlpools can be ceased.

Impel Down Gate
The gate of Impel Down is located on the border edge of the Calm Belt, so unlike Enies Lobby's gate, there is no current to prevent entrance towards the gates. However, to compensate for this, the Sea Kings that infest the area would attack any ship that is not coated with Kairoseki-hulls. The controls of the gates are located inside the generator room of the first floor.

Marineford Gate
Aside from being one of the three gates, little else is known about the one at Marineford. The Marines were refusing to open it for Monkey D. Luffy and his comrades. However, because Laffitte had hypnotized the guards to open the gates for anyone who approaches, Luffy's group has managed to leave the Tarai currents the exact moment that Whitebeard and his allies arrived to rescue Portgas D. Ace.

Trivia

 * Monkey D. Luffy has visited all three of the three places that the Gates of Justice are linked to: Enies Lobby and Impel Down and Marineford. Also, he has caused significant damage to all three places; Enies Lobby was destroyed by the Buster Call, Impel Down suffered a mass breakout and considerable damage to the front gate, and though Marineford's breakdown was from Whitebeard's earthquakes, Luffy was also somewhat responsible for causing some damage by attacking the soldiers and crashing a ship into the sea.