You have to trade.
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go down ladder and surf left, you need rock climb and go into room and it will be there and you can catch REGIROCK there if you have the special lv. 100 regigigas, it will be one of the random encounters so you just have to walk around with the event regigigas behind you and it has a 50 percent chance of it appearing.
In Pokemon soul silver and heart gold they tried not to give away to many legendaries so they decided to leave a few out including Regigigas. Although you can still get it from various cheat consoles like; Game shark or Action replay. There is one last way to get it, if you have captured him on one of the previous three games and have 2 ds' you can import (trade) him from them. Hope this answers you question ;)
Geodude evolves after level 35 in heart gold
You can't get shaymin in heart gold you have to trade it from Pokemon pearl,diamond or platinum.
Regigigas is a Sinnoh pokemon. Not a Hoenn.