We study the alien base for what it can tell us, while the center of the world we stand on is slowly eaten away.

From there, he makes his way through to an alien base.

Is there an alien base somewhere in the solar stystem?

The message reveals that the alien base is in the center of the marine's own hometown.

Thirty men and women had come to the alien base, determined to probe all its secrets.

If an alien base is found and destroyed, advance to the next level of difficulty.

What chance do you have against an entire alien base?

"We're not certain there is an alien base," said Purse.

Then, he must make a perfect landing and enter the first alien base.

Then the player must defeat the alien base and take the scientist to safety.