...this seems like a bit of a copy of some other drilling game. There's another drilling game where you upgrade your vehicle, but can play as long as you want, and when you get really deep into the earth you have to fight satan.
Anyways, this is an interesting take on the whole "hedgehog launch / grocery basket ramp" games.
The mole is so insanely powerful, if I knew it was so powerful I would have upgraded it to max ASAP. It grabs all treasure in it's path, so you can line up 3-4 items and it'll ferret them all, plus it boomerangs, so if you nitro at just the right time you can get it to catch even more treasure on the return trip. PLUS, it chews through boulders. I eventually stopped steering my drill much, and just used the mole as a seek-n-destroy / seek-n-grab missle. Seriously, once I got the mole, it was the difference between $1000 hauls and $20,000+ hauls. Plus, no time wasted chewing through boulders.
The nitro is very useful to get deep. However, don't use it first thing. Let your super drill chew through the early dirt areas where it's fast, then use your nitros about mid-way down to plow through the tougher parts. If you really want to get rich, bypass the cheap coins early on and wait until you can get really deep and mole all the gems. If pressure becomes a problem, level out and just go horizontal while mole'ing as much as you can find.
Steering & Radar are "dump stats" early on. You can go pretty far with them only about 1/2 upgraded. Your priorities should be gas/hull, then drill, then nitro/mole, then steering/radar last. Eventually you'll get so deep that you'll be raking in large diamonds and you'll just max everything else out in one felled swoop. But after everything is upgraded, it gets a little boring. I'm not much of a trophy grabber.
It's an interesting game, but it doesn't have the longevity that that other drilling game had. I sunk quite a few hours into that other drilling game, where as this one I gave with 9 weeks left since I was fully upgraded and it was boring trolling for treasure then.