Drillholes penetrating below the hot aquifer at Wairakei have encountered diminishing temperatures, suggesting that the source of the hot water is not directly helow the area now heing exploited. A magnetic survey of the field indicates that hydrothermal alteration by which magnetite is converted to pyrite extends to the greatest depths in the western half of the field, that H2S which is responsible for this conversion is therefore more abundant at depth in the west, and consequently this is the most likely feed source for the hot aquifer. The hot springs and drillholes draw on water percolating through the aquifer from this feed source.