On this page of the Mr Prepper guide, you will find information on how to get to level -4 of the Old Creek mine.
To get to the -4 level of the Old Creek mine in Mr Prepper, you need to complete missions for Joe - find aluminum foil for him in the desert, and then find his dog - Chupacabra - trapped in the forest. After completing these tasks, you will receive a red card to access level -4 of the mine.
On level -4, you can extract oil for refining into fuel and then move to level -5 (where you will find the hydrogen required to complete the rocket, and dig up valuable crystals using a digger). You can also access level "?" which is generated randomly every day – you will find there quite a few valuable items.