Printed in Analog, March 1967. Reprinted in the collection
The large structure appeared out of nowhere one day. The first man to spot it thought it might be a mirage in the heat of the day so he threw a rock at it and the rock shot back at him with the force of a .45. Since this was happening near a military missile-test site, the General running the facility contacted Richard Verner to help figure out what it was and what to do with it.