Here's a highly recommended bargain. I Dig It (iTunes link) is now only $0.99 for a limited time (was $1.99). The object of this addictive game is to dig as far into the depths of the Earth as you can using a drill. However, you must do a delicate dance of upgrading to go deeper and deeper. Fuel capacity, drill coolant, radar, and other features must be upgraded along the way, else you might get stuck in the cool deep darkness, never to see the light of day again.