There has not been a fire of this size on the Borth peat bogland for 20 years, according to Natural Resources Wales.
The Borth bogland is regarded as one of the most extensive lowland peat bogs in Britain.
The vegetation is mostly made up of purple moor grass, called Molinia.
Natural Resources Wales said that although the outside of the grass may be wet, the inside could be dry.
"The Molinia has probably been set alight by sparks or flames from damaged power lines. Molinia fires are not uncommon in dry weather in autumn and winter and can be set off by a discarded match or cigarette.
"The peat will be saturated with water and we believe that the fire has moved quickly through the vegetation without burning the peat."