I myself am not aware of this fact of finding Manaphy in Pokemon Silver, but my answer would be this cave you are looking for does not exist because Manaphy does not appear until Generation IV. If you are using cheats this answer is irrelivant.
You can get a manaphy in manaphy cave
u can only catch this in gold and silver poke go to the cave at the top north at the very end of the cave will be a BIG bump click and it will start to crack and the cave will explode you will now be outside after that walk back in where that bump was then click once more then you will then be battling manaphy the legendary Pokemon that only can lay a egg that will hatch to phione or something like that .........the manaphy you are about to battle or catch will be level100 or 1 it depends if the cave will have a earthquake that will be a level 1 manaphy so shut off and do it again if the cave explodes then it will be a madder manaphy from touching it when it was sleeping level100 manaphy was in a egg till the cave exploded it cracked open catching a manaphy will be really HARD!!!!!!!!!!! i did this method and i catched a level 100 manaphy..............
you can get manaphy by going in the secret cave also mew-to you can only get them when your champion in the Pokemon league
In Pokemon Silver, Diglett's Cave is east of Vermillion City. It can also be found north of the exit of Viridian Forest, but only if you have HM 01, Cut.
The Silver Cave is located in Mt. Silver in which you can go after you get all the Johto and Kanto badges. In the Silver Cave you can find lots of strong Pokemon and fight the final boss, Red at the top of the cave.
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It will appear in Mt. Silver cave.
You can't get into cerullean cave(the cave with mewtwo in it) on Pokemon silver because according to the game's story, the cave has caved in, but there may still be some remmants of the cave you could get to via a gameshark.
In Pokemon silver I think
Dark Cave
Silver cave in Pokemon Gold is located on route 28 on the way to Mt. Silver. The player will need Cut and Surf to navigate this passage.
After defeating the Pokemon league,Fly to the bottom of the Pokemon league and enter then walk to the left.