Explorers find a city tended by machines
Human explorers reach an alien planet and investigate an apparently deserted city whose automated systems still run, repair themselves, and respond to intruders. They eventually find the inhabitants: reduced to passive, degenerate organisms kept alive and constantly gratified in sealed enclosures by machines that meet every need and neutralize every threat. The civilization had built a perfect caretaking apparatus, and the apparatus outlived everything that made its builders capable of acting.