I just kept digging and digging. When progress slowed I sold all of my ore and upgraded my drill and repeated the process. Eventually I made it over 100km down and was earning so much money that I think I can retire. Or I could keep digging to find out just how far I can go. Luckily when progress slowed I was able to buy myself deeper.

Mr.Mine - https://store.steampowered.com/app/13...

PC Specs - Intel - Core i9-9980XE 3 GHz 18-Core Processor
2xNVIDIA - GeForce RTX 2080 Ti 11 GB Founders Edition Video Card (2-Way SLI)
Asus - ROG RAMPAGE VI EXTREME EATX LGA2066 Motherboard
Samsung - 970 Pro 1 TB M.2-2280 Solid State Drive
Corsair - Vengeance LPX 32 GB (2 x 16 GB) DDR4-4000 Memory