Any intelligent civilization would discover radio waves and begin leaking them into space.

It has led to the Drake equation, which seeks to estimate the number of intelligent civilizations in the galaxy, and listening programs using radio telescopes.

They can guide this comet throughout the cosmos searching for intelligent civilizations to feed upon.

Increasingly detailed views of the planet from Earth inspired speculation about its environment and possible life - even intelligent civilizations - that might be found there.

That can only mean an intelligent civilization.

And it's quite easy to guess one way, as Ted does, and conclude there are probably thousands of intelligent civilizations.

It is a wonder that the conception of a planned, intelligent civilization can still exist in the face of it.

The lifetime of intelligent civilizations is short, meaning the value of is small.

Dr. Drinkwater maintains that there are no intelligent extraterrestrial civilizations.

They have not found even a trace of life and Waterman thinks there was once an intelligent civilization down there.