US natural gas prices tumbled to their lowest level in three-and-a-half years on Tuesday as expectations of an unusually warm winter heaped more pain on energy producers already reeling from the collapse in oil prices. Early on Tuesday, benchmark Nymex November natural gas dropped as low as $1.948 per million British thermal units, down 5.5 per cent on the day and off a fifth from just a week earlier. The last time US gas dipped below $2 was in 2012 following a balmy winter that was the fourth warmest in recorded history. It previously traded in that range in the last century.