From Hellgatewiki.com

While the world of Hellgate: London is fictional, and almost all of the game areas are randomly-created, they are based on and modeled after real places in London.

• For a list of paths to get to each location, please see the Location Map.

Passageways

There are three types of passageways that can randomly appear in an instance. These passageways appear as trap doors or sewer covers. They are marked on the map with the same swirl symbol of exits to stations or the next instance area. The appearance of passageways is just a matter of luck. It is assumed the more +luck gear you are wearing would increase the rate of appearance of these passageways. These areas are a nice treat when found. Based on the type of passageway there are different properties associated with them. The types of passageways are treasure, hidden, and elite. It is possible to have access to another passageway appear inside of a passageway. This means you could find strings of passageways strewn together that just keep leading to each other. The passageways usually take the forms of sewers or an underground cave system with bridges.

Treasure Passageway

These passageways are filled with a large amount of palladium laying all over the ground and lots of destroyable boxes, this tends to end in a gain of several thousands palladium gained from the passage. At the end of one of these passageways there tends to be a treasure chest with a better than average item inside. There tends to only be a few mobs if any inside of treasure passageways.

Hidden Passageway

Are often just extended normal areas filled with typical mobs and boxes.

Elite Passageway

Elite passageways have a minimum of one named mob inside usually more. It seems the number is limited only by the size of the passageway, it's common to encounter one Named mob and two, sometimes three Epic plus Rares groups. This leads to an increased drop of better items from the stronger mobs. In addition there are normal mobs and boxes that would appear in a normal instance.