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The groundwater is cold, deep and twice as salty as seawater, but the water streaming out of Blood Falls, which teems with microbes, tells us that it is unlikely to be lifeless. "The fact that the [water] contains metabolically active micro-organisms that appear to be suited to life in a dark, cold brine supports the idea that life should persist throughout the subsurface," says Mikucki. If so, those microbes could be fuelling life in the Southern Ocean. By breaking down iron-containing rocks they might be dumping as much as 170 million kilograms of iron into the ocean each year, according to the researchers' estimates, helping to explain why marine productivity is seasonally very high near to the coast.

The question of cell renewal is one that all of us have intuitive daily experience with. We all notice that our hair falls out regularly, yet we don't get bald (at least not until males reach a certain age!).  Similarly, we have all had the experience of cutting ourselves only to see how new cells replaced their damaged predecessors. And we donate blood or give blood samples without gradually draining our circulatory system. All of these examples point to a replacement rate of cells, that is characteristic of different tissues and in different conditions, but which makes it abundantly clear that for many cell types renewal is a part of their story.