Spontaneous burning of bituminous shale: at the Smoking Hills in the Canadian Arctic has produced intense acidic fumigations and strongly influenced the local tundra. The burns are of great antiquity. In an area of typically alkaline ponds with pH above 8.0, ponds within the fumigation zone have been acidified below pH 2.0. Elevated concentrations of metals (aluminium, iron, zinc, nickel, manganese and cadmium) occur in these acidic ponds. Soils and sediments have also been chemically altered. The biota in the acidic ponds are characteristic of acidic environments worldwide, in contrast to the typically Arctic biota in adjacent alkaline ponds.