I only know that in Platinum If you swim in the lake beside the Battle Frontier and go upwards, You'll Reach Stark Mountain where Heatran will be available and that you can get Cresselia somehow but I don't know any Secret ones in Diamond unless you include ones that you need the AR for(Darkrai, Shaymin, Arceus ETC.) and ones that you need the old games for(Regis) P.S. I think you can still get Heatran and Cresselia in Diamond Though.
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You can only get the secret key in Pokemon Platinum
You can't rotate items. They stay in the direction you get them in.
If you are referring to secret bases, then a WiFi connection is needed.
Pokemon league champion Cynthia will give it to you when you run into her at lake valor(near pastoria and sunyshore)
yes you can if you can't do under ground you have to get an explorer kit. but when you do you have to get an digger drill. and pick aplace to have your secret base. :)