Meteor Cave? You have to recruit Lugia in Silver Trench and then Xatu will visit and there'll be a brief scene about it.
Only one Pokémon can enter Meteor Cave and you can only have three items in your inventory, not counting Hold items (and the only things you'll find in the Cave are Poké of various amounts).
Some good news is that the only Pokémon that inhabit the Cave are phantom Deoxys on the lower levels and then Deoxys itself on the last floor, but some bad news is you have to defeat at least one phantom Deoxys per floor before you can find the stairs. Paired with the fact that the phantoms are invisible at times, and you've got yourself a real pain in the neck.
So I suggest only going after Deoxys with a high level Pokémon (quoting my guide: "If the Pokémon that defeats Deoxys is at an extremely high level, Deoxys will join your rescue team."),
and have said high level Pokémon equiped with X-Ray Specs. That way you can hunt down the phantoms and get to the next floor before the 'msyterious force' blows you out for taking too long.