I finally understand what the mysterious alloy is: the power source for the pods. The ones found in the pods are depleted, because they were all used up, but the one that was found in the beginning was not.
If you go to the very bottom of the map, you will not die, but rather stop all downwards motion. It's the same with the sides, the "invisible wall" is in effect. It is rather rare to see games that motivate the player to keep on digging. Also, the usage of resources for upgrades rather that exchanging them for money is pure genius cough*motherload*cough.
I was reading about weapon smugglers in Iraq and Iran, and found that one of the most influential smugglers has died. He was called the Merchant of Death. Is this a coincident that the last boss is called "Death Merchant"?
 "Glean"){kind=icon}
 "Epic Battle Fantasy 4"){kind=icon}
 "Kingdom Rush"){kind=icon}
 "Orbital Onslaught"){kind=icon}
 "GemCraft Labyrinth"){kind=icon}
 "Little Stars for Little Wars"){kind=icon}