This achievement is unlocked once you have cleared all caves and striking locales in each area in the Aldmeri Dominion territory and unlocked the in game 'Aldmeri Master Explorer' achievement.
A 'cleared' cave/locale will show up on your map as white.
You only have to clear striking locales and caves (not any other area such as towns/dungeons).
If you go to your menu, journal, achievements, then exploration you can look through the list for "[Area] Cave Delver" and "[Area] Pathfinder" to see how much progress you have towards the achievement in that area.Striking locales:
Striking Locales show up on the map as an eye.
Simply enter the area where the eye is and it should discover it. This classes it as being cleared.Caves:
Caves show up on the map as a tilted torch.
Caves often have a quest in them, but this is not necessary to complete for the area to be 'cleared'. Instead you can ignore it and just travel through the cave until you reach its boss. The cave boss will have the special heath bar and a unique name. Once you have defeated the boss you should get an in game achievement and the cave should be classed as cleared.Maps:
These maps show every cave/locale in each area. Use them to make sure you haven't missed anything.
Khenarthi's Roost: http://www.gamemapscout.com/tes-online/khenarthis-roost.html
Malabar Tor: http://www.gamemapscout.com/tes-online/malabal-tor.html
Reaper's March: http://www.gamemapscout.com/tes-online/reapers-march.html
All credit for these maps goes to www.gamemapscout.com.