Get the explorer kit in Eterna City from the house beside the Pokemon Center. Then use it while you are outside and you'll be taken to the Sinnoh Underground.
Go underground
You have to go to the underground.
No. The underground is only in Pearl, Diamond and Platinum.
use your explorer kit
go underground and mine
you have to go underground and dig for him
you need to go underground
you go underground and get digerdrill
Go underground and start diging
Go underground and eventually you will find a Claw Fossil...
You have to be lucky and go underground and dig!
you get an explorer's kit from a guy in a hous ein Eterna city, go outside, go to key items, click it and you will go underground.