Quicksand

She appeared in our world naked, defenceless, unable to say a word anyone could understand. Her origin was at first simply a puzzle, then a scientific enigma, and finally a series of terrifying surmises that her most fascinated investigator was afraid to probe. But probe he must, for somehow he knew that this strange girl was a key to the kind of information science had sought for centuries. But the more he uncovered from the depths of her mind, the deeper became the quicksand into which his own was sinking. (First published 1967)