Astrobiologists are aware that extraterrestrial life might differ from known life, and considerable thought has been given to possible signatures that might attach to weird forms of life on other planets. So far, however, very little attention has been paid to the possibility that our own planet might also host communities of weird life. If life arises readily in earthlike conditions, as many astrobiologists contend, then it may well have formed many times on Earth itself, raising the question of whether one or more shadow biospheres have existed in the past, or still exist today. In my talk I shall discuss possible signatures of weird life, and outline some simple strategies for seeking evidence of a shadow biosphere.