Upper Relic Castle
Relic Castle. Ruins of ancient structures for people to scour and scavenge. First thing is first, you cannot run on the quicksand, otherwise you will sink. Same goes for going in the middle of the quicksand, so walking past the edges is a good idea. If you walk past the edge of the one by the stairs, you can go down and get TM 39: Rock Tomb. This 50 Power, 80 Accuracy move lowers speed if it hits, but it is really only good for Technician Pokémon like Breloom in this Generation, and the accuracy is not really worth it. If you really want an 80% accurate Rock move, you may as well go with Stone Edge.
Going down the quicksand lands you in, surprise, piles of sand. It is pretty much the "try again" room of the building, but it is better for running into wild Pokémon in here. There really is not much else to look for in here, because we cannot access anything else from this side (sound familiar).
There are new Pokémon in here, however. Yamask is a bulkier Ghost type that has an ability that makes it so if the enemy comes in contact, they lose their ability. It is a neat gimmick, and can earn a spot on a team.
Looks like there's no other option but to turn around to Nimbasa City.