Pokémon Diamond and Pearl/Oreburgh City

Oreburg City is a coal-mining city, home to the first Sinnoh region Gym Leader - Hyōta. It also houses a museum which revives the fossils of ancient Pokémon

When you first enter the town your rival will block the entrance to the gym. Then go south to the mine. Then go through the mine till you find the the leader he will then go back to the gym. And the next time you visit your rival will be gone and you can challenge the gym