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.
You have to go to the underground.
Go underground
No. The underground is only in Pearl, Diamond and Platinum.
go underground and mine
you need to go underground
use your explorer kit
you have to go underground and dig for him
you go underground and get digerdrill
Go underground and start diging
You have to be lucky and go underground and dig!
Go underground and eventually you will find a Claw Fossil...
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.