Second Landing by Murray Leinster — Read

The Elektran solar system displays certain anomalies, not only in / the existence of a satellite-sun Rubra, no larger than a gas-giant / planet. (but in) the twin worlds Thalassia and Aspasia, each / nearly seven thousand miles in diameter, which revolve about each / other at a distance of only 250,000 miles. Tidal strains have long / since ended their diurnal rotation and they turn the same faces / toward each other during their period of revolution of not quite 25 / days. This nearness and the development of intelligent races on / both planets led to the development of interplanetary communication / between them some time between 7,000 and 11,000 years ago. The / tragic results of this communication -- _Astrographic Bureau / Publication 11297, Supplement to Space Pilot Vol. 460, Sector XXXIV, / p. 56._

A trenching-machine with its buckets removed went toiling painfully / up to the alien tripod some six hours later. It was under remote / control. It skirted the elongated opening of a concrete tunnel, made / by the long-dead, six-fingered race of which the exploration-ship had / found skeleton remains. There were thirty or more of those tunnels, / which of course no member of the Expedition had yet entered. But the / _Franklin's_ report said that the tunnels had been launching tubes / for giant rockets. The rockets had gone roaring out over the ocean, / rising steadily, until they swept round the curve of the planet / to blast across space and loose destruction upon the sister-world / Aspasia. The firing-plaza took its name from these tunnels. The / refugee-settlement -- shelters of lignin plastic -- had evidently been the / shelter in which the dying, despairing Thalassians lived while they / took their revenge.