This area is underground of Rock Ridge and is not a dungeon, thus can be memorized with Acolyte's Warp Portal.
There are 2 ways to enter (must be lvl 100+ for NPCs to allow you through):

1. Talk to Joe (harboro1 324/124)Paste Ingame at the east side of Rock Ridge, on top of stairs of a house to enter.

   

2. Talk to Underground Passage (harboro1 335/144)Paste Ingame that is near Joe, under a water tower to enter.

   

There are 2 ways to exit:

1. Talk to Underground Passage (harboro2 331/187)Paste Ingame at the northeast side of the map.
   

   

2. Enter the portal (harboro2 283/104)Paste Ingame at southeast side of the map.