A deep-sea core from the Caribbean contains a layer of sediment highly enriched in meteoritic iridium. This layer underlies a precisely deated (34.4 m.y.) layer of North American microtektites and coincides with the extinction of five major species of radiolaria. A causal relationship is strongly suggested: a massive (⩾3 km, ⩾50 billion ton), chemically undifferentiated meteorite collided with the earth, produced the tektites, and led to worldwide extinctions 34 m.y. ago.