Gates of Justice

The Gates of Justice (正義の門, Seigi no Mon) are three massive structures that each located in 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 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. While World Government affiliates can travel in and out of the Tarai currents to approach one of the three destinations, if criminals somehow managed to enter it, they would most likely be trapped, as 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 Marine Headquarters or Impel Down. Once a criminal passes through the gates, there is no longer any hope left for them.

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, the 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. However, the Marines are currently refusing to open it for Monkey D. Luffy and his comrades. But someone inside Marineford has opened the gate at the exact moment that Whitebeard and his allies arrived to rescue Portgas D. Ace.

Trivia

 * So far, Monkey D. Luffy has visited two of the three places that the Gates of Justice are linked to: Enies Lobby and Impel Down. Also, he has caused significant damage to both places; Enies Lobby was destroyed by the Buster Call, while Impel Down suffered a mass breakout and considerable damage to the front gate. As Luffy is nearing Marineford, if he were to successfully pass through the gate, this would mean Luffy has visited all three places.