You can get the Marking Map app from the Pokétch Company after defeating the Hearthome Gym leader.
At Sunyshore.
NO THERE is no second map in diamond and pearl.(that includes platnium).
Go to Eterna City's Pokemon Center. Talk to the girl.
It is at the bottom of the map where it says Diamond Mines.
They are not called diamond places first of all! It is called the diamond mine. You get to the diamond mine by going on the map and flying there. You get the map by saying you will try to find the gems for the Nabooti statue. HAVE FUN ON POPTROPICA!![:
The Diamond Mines are at the bottom of the map, in South Africa.when you get the map go to the bottom right corner then go left.
use your pokeapp
There is a map of Mt. Coronet in the Sinnoh region.
use a marking map
it is at the top right of the map
There is a map in the Pokemon Diamond/Pearl Guide Book.
Follow the directions in these pictures: http://bulbapedia.bulbagarden.net/wiki/Turnback_Cave it worked for me in the diamond and pearl games and even in platinum. Good Luck ;)