Archford

Originated in PMU 6, Archford is a region accessible in PMU 7, though one must use HM Surf from the Exbel Penninsula and beat Crescent Islet near the South Outpost Island first. Archford has its own dungeons, non-player Pokémon, Housing Center, and town. You can fly down to Archford from the lower left area of the sky. It is an overgrown plant region with many dungeons and new Pokemon; as it's the last region that can be unlocked, it is considered the hardest of the four available regions.

Dungeons

 * Mt. Moon
 * Mysterious Jungle
 * Boggy Wastes
 * Murky Trench
 * Voltaic Rainforest
 * Inferno Volcano
 * Stark Volcanic Woods
 * Starlit Lagoon
 * Seafloor Ruins
 * Sandy Shallows

Underwater
In Archford, using any of the dive spots in the sea will take you to its unique Underwater subworld. Wild Water-types accompany it, and you can return back by stepping on any of the landing tiles via first using Dive. It's only purpose as of now is to hold the entrance to Seafloor Ruins.

Video
The original way to get to Archford in PMU 6 is by taking the Ferry from the Winden Docks.