Turnback Cave is a cave in Sinnoh that can be found by going to Sendoff Spring, which is located past Spring Path. The player character can only go to Spring Path if they've received their National Pokédex.

Where is Giratina in Turnback Cave?

Turnback Cave is located at the north-central region of Sendoff Spring, which itself may only be accessed through the Spring Path. In Diamond and Pearl, in the room Giratina resides in is an inscription that implies the cave is a location where the separation between the real world and the Distortion World is weaker.

When can you access Turnback Cave?

You can enter Turnback Cave after you beat the game. The first room has a large pillar at the center. Read the guide written on the pillar to learn how to get to the final room. Actually, you need to pass the room that has the large pillar at the center three times before you pass 30 rooms.