Travelling the Indus valley late in the third millennium BC you would have been awed by two Bronze Age megacities, 320 miles apart, ‘massive and tightly planned, very similar in layout’, their bricks and measures standardised, evidence of rigid authority. Their trade goods included Afghan lapis lazuli, Omani vases, legal seals from Sumeria, carnelian beads, packed for dispatch to Sumer — and that is almost all that is left of Harappa and Mohenjo-daro, and more than we know of them.