Down the coast from Blenheim, a large shallow lake has ponds that develop a deepening pink colour during the summer months. At the same time, huge white mounds appear on the shore. What on earth is going on here?This fascinating and rather alien landscape is the result of natural salt production. Seawater, fresh from the Pacific Ocean, is pumped into Lake Grassmere. Warm north-west winds blow across the exposed lake, evaporating water and increasing the concentration of salt. The very salty water is pumped into deep holding pens, then into shallow crystallisation ponds. As the water continues to evaporate, salt forms as a crust on the bottom of the ponds. The remaining water is pumped out and the dried salt is harvested, crushed, washed and moved by giant conveyor belts to form huge mounds of sparkling white crystals.The pink to purple colour of the crystallisation ponds is caused by natural microscopic green algae that change to pink in the high salt concentration. The same phenomena gives the Red Sea its name. There are also small pink shrimps in the water that thrive in this salty environment.Other salt works in the world are generally much closer to the equator, but Marlborough's abundance of warm north-westerly winds, long hours of sunshine and low summer rainfall provide the evaporation needed to extract salt from the sea at this latitude.