It actually is the entrance to Barheim Passage. Your stay there has been ended with passage collapse. There is no return, right? False. There is another passage north of this collapsed one. Only thing you need is a key.
You can take up this mission if you have finished the hunt for Flowering Cactoid (rank I).
After a winning fight with Flowering Cactoid and collecting the prize from Dantro talk with him once again. He will ask you to take a flower Cactoid to his wife to a village on the south bank of the river (South Bank Village). She will have a few tasks for you. After completing all you will be awarded with a key.
The first task is to collect shells: Sem-clam shells. Leave the village and go west. Get into water and look for flashes [screen 1]. When you collect all, come back to village and talk with Dantro's wife.
Now, she's going to ask you to bring her Nebralim. Go to Dantre and talk to him . He says there is a little nebralim in chests nearby. Approach them, take nebralim [screen 2] and go back to a village to talk with Dantro's wife.
Now, You won't be able to continue unless you have come back from Raithwall's tomb.
Come back to village where was Dantro's wife. You'll see that Tchigri's dad's boat have returned at last [screen 1]. Recently he hasn't been returning for a long time. Go and talk with Tchigri. He will tell you that his father didn't come back. Lad wants to search him but seniors are concerned abort him so they ask you for help. Pick: "Let's go" and swim to the other side. There, go and talk with Ruksel who stands by the fireplace [screen 2]. Now, you gotta come back and talk with Dantro's wife. She will tell you that a flower which you gave her is behind the house.
Go there and pick up flower from the ground [screen 1], swim to other bank and approach the group of Cactoids standing there. They will think that this cactus is actually their brother and will go away. You will get 1000 gils and Wyrmfire Shot. Now you can go to the northernmost part of Westersand: The Broken Sands.