Speak with
Bob

Follow the arrow and turn left before entering cave to find area to get Sand.
Use the

to dig here, for the

Collect 4x Sand if You are doing Quest alone, 5x if with friends!
Use sand here

Enter stairs do go down and follow the route to find button

Push the button and go near closest Exit. If doing this quests with a friends, all other must stay down and follow the route to find bridge end.
Enter the place which was previous blocked

If You are doing this quest alone, enter the stairs
If Your friends are waiting, use sand here

Speak to
Jolene to finish the quest.