its in blackthorn city behind the gym
The Dragons Den is behind the Blackthorn City Gym. You just surf around in the water behind it.
There is a lake behind the blackthorn city gym. Use surf and go behind it and you will see a small cave called the dragons den
You dont.
beat the last gym leader on soul silver in johto
you have to goto the dragons den after u beat Clair
you do ecruteack city
Blackthorne city is not in Pokemon Diamond but in Pokemon Heart Gold/Soul Silver
in ecruteak city
goldenrod city.
it is in the city after pewter
By going to saffron city.
it is in goldrod city