The Clones of Mawcett (A Galaxy Unknown, Book 3)

While digging through the ruins of a long dead civilization on the planet Mawcett, a team of Terran and Nordakian archeologists find an underground tunnel leading to a door in such extraordinary condition that it could have been fabricated as recently as that morning. They labor throughout the day, using every modern device available to them, but are unable to gain entry. Then, in the early hours of the new day, seemingly in response to the screams of a frustrated, over-tired scientist, the door slides noiselessly open.