Thank you!
I totally get what you mean. Perhaps I should have included another mechanic a few levels in or something. The idea for progression is that more of the basic mechanics of the game get taught to the player through the levels, even though they're in there from the start. The big mechanic being that the closer you are to the center of the planet, the faster you are to the other side (thanks high-school mathematics)! However, another mechanic, like different types of soil that are harder or easier to dig through, would have added a ton more possibilities and progression.
Thanks for your honest feedback, I appreciate it! :)