Pokemon Heart Gold and Soul Silver - Walkthrough - Route 41 Revisited Pokemon Heart Gold and Soul Silver: Walkthrough - Route 41 Revisited
Basically, if you SURF from north CIANWOOD, skip the first WHIRLPOOL you see, keep going right, then down, and it will be that one.

Once you cross over it, make your way into the entrance.
This walkthrough was written by MKaykitkats, used with permission.