Splashdown In Fiend's Fen

From the introduction: A month ago, a strange object plummeted from the sky and crash-landed in the heart of Fiend's Fen. Now it lies partly submerged in the boggy ground: a crystalline cube, 50 feet on a side, pearly white but tinged also with a weird, unearthly hue. At night, it glows softly. A constant, low-pitched hum emanates from the alien spacecraft.