BEFORE the development of the blast furnace, iron smelting was achieved by ore reduction at temperatures below the melting point of the metal, forming an agglomerated 'bloom' of low-carbon iron and slag. The forced-draught (bellows-operated) shaft furnace known from archaeological studies is usually regarded as the pinnacle of this early smelting technology1-3. Examples of natural-draught furnaces, in which gas buoyancy in a shaft of sufficient height induces a draught adequate to drive the smelting process4, are also known, but are generally regarded as disappointingly inefficient by comparison5. Here I report the discovery and excavation at Samanalawewa, Sri Lanka, of a previously unknown furnace type. The furnaces are all situated on the western margins of hills and ridges, where they are exposed to the strong monsoon winds. Field trials using replica furnaces confirm that this furnace type uses a wind-based air-supply principle that is distinct from either forced or natural draught, and show also that it is capable of producing high-carbon steel. This technology sustained a major industry in this area during the first millennium AD, and may have contributed to South Asia's early pre-eminence in steel production.