You will need a Pokemon with the move Flash to light up the cave.
If you have Emerald on visual boy advance though, you can use this GameShark code:
Full Light in Dark Areas
0C7BD341 E9775222
I'm not sure if it works for other Pokemon versions.
You have to go to the top of Bellsprout Tower and defeat the elder, in which you will get the item Flash. This move allows you to light up any dark cave.
Up your moms @$$
When you get to violet city go to the pokemon center go left then down then you will come up on a place called route 32 and at the end you will find a pokemon center and a cave and thats union cave.
it's either a cave or the area of a shiny Pokemon
you go up to them then push them
You have to go to the top of Bellsprout Tower and defeat the elder, in which you will get the item Flash. This move allows you to light up any dark cave.
use the hm flash which is given by sage at the top of sprout tower
u need to get TM Flash and a Pokemon who can use it and then go to the cave and go open up the menu go on Pokemon then go on the Pokemon who uses flash then press the A button on then then press Flash.
You can either find one in Veilstone where you can buy one, or get a free one in the Orebourgh Gate. When you're in a dark cave, go to the menu screen, select Pokemon, the select the Pokemon that knows flash. Then press the button that says flash. The whole cave will light up and stay lit up until you exit the cave.
All you need to do is: talk to the man in artitian cave, and he will give you HM05 (flash). BUT, you can only use it after you have defeated the second gym leader. Once you have, go to the cave's basement, and as you can see it is scary dark. Go and teach the HM to a Pokemon, then press START, and then POKEMON, and go to the Pokemon that knows flash, press A on it, and press flash. As you will notice, the cave will light up. You can not light it up all the way as seen in the older generations of the Pokemon games... This can be used for all caves that are dark. Well I hope this message helped you loads. :)
after you beat the Pokemon league go where the shoal cave is then look to your left and the is a dive spot dive and go through and then go up then there is a cave and that is the marine cave.
Up your moms @$$
Go to the 6 badge city and go down , take the ship and then go to the cave , in the end of the cave is the light stone .....
Teach it to a Pokemon, then select the Pokemon you taught it to in the party and select Flash.
if you catch a Pokemon with a dusk ball in the light or at day time when your not in a cave, half the time it will be a shadow Pokemon. i did this with my hippowdon.
When you get to violet city go to the pokemon center go left then down then you will come up on a place called route 32 and at the end you will find a pokemon center and a cave and thats union cave.
Youse can't. It be impossible. U got to trade into Pokemon platinum version and lvl it up at a certain cave, but me forget what cave it be. Sorry. You level it up in Mt. Coronet / Chargestone Cave. - Cooldude335 -