Where is seaded cavern?

If you mean Seafloor Cavern, you get to it during the initial game when Joe Wildwest asks you to go to there to stop the islands from sinking by either placing a charge or finding a charge and defusing it, I forget which. This is so far the only way I know to get there and it's only one time during the game. If you haven't beaten the main game yet, keep playing, you'll get there eventually. Make sure your sonar is completely, and I mean Completely! Upgraded. Try to have an empty fossil case as well. Scour the dig site and get ALL the fossils you can find before completing the task. Once you leave, any fossils you didn't get that the official website lists as coming from Seafloor Caverns will be scattered over the other dig sites in the game as "specials". They will be hard to find, but doable if you're persistent. If you ever do find a way to revisit the Seafloor, please post it-you'll make a load of people very happy. If you have the original game and access to a second DS, you can get any vivosaur from 1 to 100 in the original game and use the fossil cannon to shoot the fossil rocks over to your new game. That's what I am doing, since if the heads are hard to find, the rest of the body parts are almost impossible to find in the Champions game. Good luck. Hope this helps.