Dig Deep

To unlock diggers you first need to build the house on the far left for 100 coins. Coins are earned through digging and trading the purple balls to the guy at the top. Once you've built the house, walk up to it and it will open a menu where you can rent a digger. You will need to buy:

John - 100 coins
Paul - 100 coins
Hana - 100 coins
Cindy - 10 gems

Gems can be found by digging below depth level 10.