Bottomless Pit
by Philip E. High, New Worlds Science Fiction 128 (March 1963), 49-63.
High’s fiction often gave creative vent to anti-militaristic sentiments, favouring fanciful SF means of triumph over unwarranted aggression. This short story posits a planet near-mythical in fending off alien invaders. Terrans are the latest, falling foul of a unique if improbable ecosystem.