blue colour geyser hot hot springs minerals nature orange sulphur thermal pool volcanic water yellowstone bush geothermal leaves new zealand plants Rotorua steam thermal tree Waiotapu billies billy conserved-heat container convenient cooking-device durability durable economical efficient fuel-minimises fuel-minimizes handles heat-loss-minimised heat-loss-minimized heat-retention ice-cooler inner-pots insulated insulation keeps-flavour less-evaporation lids non-electric pan picnics portable portable-oven pot pots retained-heat retains-nutrients rice-cooker safe safer sauce-pan slow-cooker spill-proof stainless-steel strong thermal-cooker thermal-cooking thermal-insulation thermos-vacuum travel vacuum-cooker vacuum-flask water-saver zero-energy-use channel hot spring water flow yellow thermal pools