Get to Giratina and once the battle is over, the exit should be downwards. If not, look around. Alternatively, use an escape rope. :)
The turn back cave in Pokemon Platinum is in Seodoff Spring near veilstone city
in turn back cave
Look in related links to this question
you get girantina in the different dimension in turn back cave
You can find dusclops in turn back cave.
In turn back cave
in Pokemon platem tern back cave is in side of sprind path wall
its on the spring path in between pastoria and veilstone
you find it at spring path by turn back cave
You need TM Flash
there is no cave of time
Hippopotas is located in the Ruin Maniac cave in Pokemon Platinum.