the dark cave can be entered on route 45 or route 46
Dark Cave
Dark cave.
You can find Waterfall in the Dark Cave.
in the dark cave near blackthorn city
if this is the swirl islands . . <-- . . go to this one
In Gold and Silver Wobbuffet can be found in Dark Cave. In Crystal, in addition to Dark Cave, you can also get one at the Goldenrod Game Corner for 1500 coins.
In either the dark cave or the cave leading to azelia town, I don't remember, but you have to go deep into one of them and get the rarest water Pokemon (except for lugia) its Lapras.
In Pokemon
You can find it in the Dark Cave or at the Goldenrod Department Store for 2000
just look the walkthrough on youtube
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.
You can catch gastlys in all of the following areas: Sprout Tower Burnt Tower Union Cave Dark Cave Hope this helps