Pokemon White Cheats[ NDS ]
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Secret - Abyssal Ruins
In the Abyssal Ruins, which can be located at the eastern side of Undella Town (where you can battle Cynthia during Spring-time). Now, in order to go there, you are being required to have the following:
Next you may have to follow the guide here. Credits goes to Marriland for the useful map.
Now, in Abyssal Ruins, as you first entered it (recommended to start at the lower-left end as you dive), you will notice a faint sound. But you do have a step limit though. The more you traverse long enough, you will be sent back to the surface where you last dived from. What you will find in the Abyssal Ruins are Relic treasures and Arceus' Elemental Plates. Maybe some other items as well.
On 1F, you have to go through the path correctly (as shown on the map) or else you will not able to unlock the first block. You can, however, go pick up some loot for your own satisfaction. On 2F, you are being required to use Flash on any of the two blocks (as shown on the map). If you don't have a Pokemon that knows Flash, then collect the loot as you wish. Now, on 3F, at the last block, you are required to use Strength. Again, if you don't have a Pokemon that knows Strength, just collect the loot. And finally, in the center of 4F, you will find a RELIC CROWN! It can be sold for an extremely high price.
If you have collected so much Relic items, they are to be sold at the villa in Undella Town to the Black Belt. He will give you a heck lot of cash for them. Unfortunately, there's only one Relic Crown.
Enjoy your search in Abyssal Ruins.
Edit: Because that the link to the map I was talking about is not there when it was first submitted, please stop being so dramatic and shut up for a moment. The link to the map is here. If it still does not show up, then you will need to go to Marriland's Site for the map.