The cave is for unlocking special pokemon and to give mistery to the game. Pokemon:IN DIAMOND if you defeated dialga you can battle it again there if you have the lakes pokemon.(in pearl is just the same but with palkia).IN PLATINUM you can get a special pokemon but with some special conditions first in the story when you battle giratina don't capture him just defeat him then beat the game. After you beat the game you can get to the turn back cave and you will have to catch giratina with a premier ball (its very difficult, its like catching it with a pokeball be prepared).it will be at lvl70 I think. Then capture dialga and palkia at lvl 70 with a premier ball.then capture the lakes pokemon with a premier ball (lvl 50). Don't lvl up any of this pokemon( the space time and distorsion legendaries at lvl 70 and the lakes at 50) then go to celestic town first go with the lakes pokemon(and no other pokemon) to the cave talk to the painting then go out go again with the space time... Pokemon (and no other pokemon) and again talk to the painting then go with the six pokemon and talk to the painting and a special pokemon should appear in the middle of the cave.I don't say which pokemon it was because I did it by an accident (I tried catching those pokemon with premier balls and then I heard that of diamond and I tried it to see what hapened I got it this way but if someone got it different please tell me)and I told my friend and he lend me his game and I did it on his game and it was a different pokemon( I think it was because mine was a spanish version and his was in english. Hope I helped.
