ok let me make this a little clear with some explanation and I hope you get it. Lithium ion batteries are filled with lithium(pretty simple). Lithium is one of the most violently reactive chemicals on the periodic table. It is so reactive that when it is exposed to air it reacts with the oxygen (combustion aka caching fire) in the air to make Lithium oxide. Otherwise it is nicely sealed(away from oxygen) but since that you have made a hole in the seal, the chances of catching fire are dangerously high. And so to dispose it I suggest that you drop it off at a battery disposal site in your town (radioshack/ the shack people usually participate in disposal programs) and if you can't find any throw it away where you think if it explodes it will not do any damage, for example as you said before bury it deep.... less O2 inside the ground less chances of explosions. Or last, look up on google, there must be some other proper guidelines.
Have a good day.
