On each level, your search for a bone is the key to going down. The map does not change, so each time you go to the surface to recharge you need to go to the bone on each discovered level. Ninja Adventure requires practice and a good memory.
Use the money you earn to buy power-ups. Initially, the best plan is to buy bags to carry more and batteries to last longer each trip. Upgrade your digger. Better diggers can get the treasure out of the hard rock. Also, each digger allows you to skip getting the bone on an upper level.
It is all about finding the bone. The bone is the key to unlocking the next level. The bone does not move between trips or games. Use your memory of earlier adventures to get to the bone more quickly. It is not as easy as you think.