Destroy the control center at the core of the Earth!
In South Park the aliens had a probe installed in Cartman's... lower back side! In this game the aliens, the Zybonians, as they are known as, moved up the ladder a notch, and managed to ensnare the entire human race, by installing a control device at the core of the Earth planet! Now, as you can imagine, your goal is to go deep down, in the core of the Earth and take this alien signal transmitter and destroy it. Which is easier said than done, but it's nonetheless doable. That is so because Deep II: The Center of the Earth does a great job at it, even if it was a game programmed entirely in Turbo Pascal, a good programming language, but definitely not something that was cutting edge. Thus, the graphic framework is made of monochrome squares, which are by no means anything more than just, well... functional. But then again, the game is more of a racer/fast paced puzzler, so that won't create any issues with you. So, overall, Deep II: The Center of the Earth is worth it, if you played Deep back in the day and want something of the same build, but different enough not to feel like a total rip.