You need to get and teach HM Flash to a Pokémon.
Teach one of your pokemon 'flash' using a TM/HM
If by Underground Caves you mean caves where you just have a small radius of light around you, then you talk to the Hiker in Dewford Town caves and get HM Flash from him. Beat the Dewford Town Gym Leader and teach Flash to a suitable Pokemon. When in these caves press the START button and activate your Pokemon team and right click the Pokemon you taught Flash to and activate Flash on the List 1. Talk to Dewford Town Caves Hiker 2. Beat Dewford Town Gym Leader 3. Teach Flash to Pokemon 4. Use in Dark Caves. Viola!
Teach a move to a Pokemon that can learn it via HM and then, after defeating Falkner in Violet City, select that Pokemon in your party and select Flash to light up dark caves, such as Dark Tunnel.
get the item flash
As far as I know you can only get it with Pokemon that already have it. But i expect you could find one in dark places like caves.
Teach it to a Pokemon, then select the Pokemon you taught it to in the party and select Flash.
Use the HM Flash this is given by the sage at the top of sprout tower.
caves caves caves caves why do u want to know?
Dark Pokemon only exist online. you've been searching for nothing. sorry. '-'
how do you use flash? just go to pokemon and go to the pokemon that knows flash. press A and it will say in blue text "Flash" select that and it will use Flash. it's very useful in dark caves.
in grass, on water and in caves.
caves