Meteor 2 Easter Egg - Doom tank in level 1

Go to the lower left corner of the map. Touch the skeleton. You will hear some music, and a doom tank will appear along with some enemies. Also, if you check the script of that level(you can find it in the maps folder) you will see a function called:

ReleaseTheDoomTankThatIsHiddenInTheBottomLeftCornerByTheSkeleton.