Perhaps the Earth was not yet ready for contact with alien civilizations.

They're planning to wipe out an alien civilization to take what we need.

The chance to see a real alien civilization is too much to pass on.

The alien civilization that created them was long gone, but their work lived on.

The alien civilization was an advanced one, and the computers were very sophisticated.

Certainly the alien civilization was a deadly unknown right now.

The failure to communicate with an alien civilization is the main theme of the book.

Perhaps he was the first human to glimpse an entirely alien civilization.

It is a story of a first contact between human and alien civilizations.

She was looking for truly alien civilizations, but found nothing, and at last decided to return.