The upper image shows phosphate nodules on the sea floor; the lower image is a close-up taken in the laboratory. At depths of 400 metres, phosphate deposits occur along 400 kilometres of the Chatham Rise. Dredging trials have estimated that the resource amounts to 100 million tonnes of nodules, averaging 21% phosphate. Typically nodules are 1–4 centimetres across and form a layer up to 70 centimetres thick, within a sandy bed. The nodules are patchily distributed, with the highest concentrations averaging 66 kilograms per square metre.