It could be an Elizabethan witch's totem or a contemporary manifestation of wicca. Either way the crude wax figure fished out of the Thames by American artist Mark Dion is creepy. It lies in a drawer in the Tate Gallery along with charred animal jaws and bears' claws, a rusty chain attached to a pole that you imagine might have been used to drown river pirates, and a medieval scabbard - which, on closer inspection, turns out to be a child's plastic toy. As an authority on the Thames and its history, Dion, who spent the summer foraging outside what will soon be Tate Modern and Tate Britain, is even more unreliable than the guides who wing it on tourist riverboats to Greenwich. The objects he discovered are not classified by period or labelled, but arranged according to an aesthetic taxonomy all his own, in the Tate's Art Now room. The Thames is being rebranded for the millennium, but Dion echoes Marlow in Conrad's Heart of Darkness who says that 'this also has been one of the dark places of the earth'. He invokes the river's secret histories, anonymous, forgotten dramas. How is this art? Dion trained as a natural scientist. His installations are 'research'. Here it is the way we imagine ourselves in history that is in question. Dion's installation removes the rational structures - dates, narratives, names - we impose on history to keep it at a distance, to turn time into history. This is the undertow of millennial London.
• Until February 27