They must be the work of some unknown prehistoric race.

Then the unknown race disappeared, leaving only parts of their technology behind.

The System was created millennia ago by an unknown alien race.

He has no children, and it is prophesied that a woman of unknown race will bear him a son.

In which case, we are dealing with a previously unknown race.

The other 23 included 4 women, 7 blacks, 8 whites and 6 of unknown race.

However, at no point was the leader of the race unknown or disputed.

This building was in good repair, as is often true of the constructions left by that unknown race.

As the situation became out of control, an unknown race appeared.

If we've been captured by aliens of an unknown race, we can't take anything for granted.