The Sunyaev-Zeldovich effect detection towards 1643+46, a pair of z=3.8 quasars separated by 198 arcsec (plotted here as crosses). Contour levels are -325 to +130 µJy in steps of 65 µJy; dashed contours are negative. The implied gas mass exceeds 2 x 1014 M (or a total mass above 1015 M but there is as yet no corresponding detection in the X-ray waveband, or an obvious rich cluster in 4-m near-infrared images. It is hard to escape the conclusion that this is an ultramassive system at z > 1. Optical and near-infrared follow-up will only be possible with telescopes with the light grasp of Gemini.