An easy way to get a lot of mileage is to do Pressure Mode and dont kill the robot instaed just keep drilling farther and farther until you die and if you die then start all over.And reapet the process.I got about 10,000 mileage in 20 minutes.
Endless Mission Mode
To unlock the Outer Space (Infinite Depth) Level in Mission Mode, you must gather all the characters from the regular stages (hence beating Mission Mode).
Mirror mode time attack
Beat time attack letters A through J to unlock mirror mode.
Mr. Driller Badge
Successfully complete a Mission Driller mode level without using any items or losing a life. The Outer Space level does not have a badge since it is endless.