The southwestern caves are located/accessed in the Shadow Fort. After joining the Wyrm Hunt you are sent to the Shadow Fort to assist in retaking it from the goblins. After sneaking inside and fighting the two Ogres in the center courtyard there are 3 holes in the courtyard. 2 are too small to get into (goblins spawn here) the third is large enough for your party. Jump down and follow the tunnel until the achievement pops.