These slicks are made up of masses of blue-green algae called Trichodesmium. The algal cells can join up to each other in strings and clumps. As the cells age they become positively buoyant and rise to the surface. If it is calm, these cells get the opportunity to aggregate into huge slicks that are sometimes so vast they are visible from space.

Unfortunately, the full significance of this phenomenon remains poorly understood because it is so tricky to study. Trichodesmium is impossible to culture for laboratory studies, and effective field observations are rare. While research efforts are underway to redress this, the most useful observations remain those made by Captain Cook over 200 years ago!