Deep Blue Sea is a hat designed by Australian milliner Ann-Maree Willett, made from Australian wool felt and set with 26 precious opals from Lightning Ridge, weighing a total of 1447.5 carats. Deep Blue Sea was the feature piece of the ‘Climate’ series of hats that launched Willett's international millinery career. It contains more Lightning Ridge precious opal than has been brought together on any other wearable item and became famous as “the million dollar opal hat”, then later as “the most valuable hat ever offered at public auction.”