Satan

Satan is the final boss of the game and found at the lowest level of Mars, having previously disguised himself as the benevolent Mr. Natas.

When you find (and fall into) the hole at the bottom of Mars, Satan reveals he is the one who has been killing miners that dared venture into the bottom. After his true identity is revealed, the fight begins.

For details for the encounter and aftermath, see Ending.

Trivia

 * His hellish domain is shows up on the altimeter to be -66666ft, in reality it is about -7400 ft, with -7287 ft being the maximum depth diggable (walled off battle area accessible only at the far right side of the bottom). This is a reference to the number of the beast in real life, and along with "Natas" being "Satan" spelled backwards and his eyes being red even in human guise, are hints to his true identity.