Very fun game once you get some upgrades going. The engame farming might be tedious, but for me it was really fun digging down to unseen places and getting surprised by treasure or big gems. The highest upgrades are kinda pointless at some point though, not even worth upgrading them all to max, because you don't need to anyways. (I only maxed drill, to get the last treasures without using dynamite)
As others said, it would be fun if there is a different ending, or in fact, an end at all. After collecting all the treasure and medals, it's just kinda over. You could do something with the black box for example.
In the game its said to contain mysterious things for scientists to discover and research about, maybe you could build such a research center with money, or upgrade a scientist where you can fund its research to unlock some kinda ending. Maybe like after you finished researching the black box, it contains a hidden entrance to some kind of portal, or maybe you can even exceed the lower ground limit to find like a huge alien ship or dinosaur or something.
Other than that, I really enjoyed playing through this game and spending countless hours on it =)