There are many special items there, ending in the Relic Crown. You have to swim around, and you get a time limit. YOu can make it to the end eventually...
Relic Copper (x6): $1,000
Relic Silver (x6): $5,000
Relic Gold (x7): $10,000
Relic Vase (x4): $50,000
Relic Bracelet (x5): $100,000
Relic Statue (x3): $200,000
Relic Crown (x1): $300,000
You can sell all of your loot for a combined price of $1,706,000!! That's a ton of money, but don't worry, because unlike previous games, you can have more than the usual $999,999 in your wallet. Go ahead and sell whatever you'd like of it, or you can always trade it to a friend or another version of yours to give it some extra funds (after they've gotten to Undella Town, of course).
From Marilland. You can see here for more info.
