The liquid metal mercury is classified as a volatile element, one that lives mostly in the Earth's crust. Its geochemical cycle starts with volcanic activity as magma invades sedimentary rocks. Mercury vapors and compounds rise toward the surface, condensing in porous rocks mostly as the sulfide HgS, cinnabar.

Cinnabar from the Almad�n mine, Spain.

Hot springs can also concentrate mercury, if they have a source of it down below. But deposits of all kinds are usually small and rare. The delicate element doesn't last long in any one place. For the most part, mercury vaporizes into the air and enters the biosphere.

Only a portion of environmental mercury becomes biologically active; the rest just sits there or becomes bound to mineral particles. Various microorganisms deal with mercuric ions by adding or removing methyl ions for their own reasons. (The methylated mercury is highly poisonous.) The net result is that mercury tends to end up slightly enriched in organic sediments and clay-based rocks like shale. Heat and fracturing release the mercury and start the cycle again.

Of course, humans are consuming large amounts of organic sediments in the form of coal. Mercury levels in coal are not high, but we burn so much that energy production is by far the biggest source of mercury pollution. More mercury comes from burning petroleum and natural gas. That's just one of the unrecorded costs of fossil fuels.

Mercury used to be highly regarded, for reasons both mystical and practical. Among the substances we deal with in our lives, mercury is pretty weird and amazing. The Latin name "hydrargyrum," from which its chemical symbol Hg comes, means water-silver. English speakers used to call it quicksilver, living silver. The medieval alchemists felt that mercury must have a mighty mojo, some excess of spirit that could be tamed for their great work of turning base metal into gold.

As a practical matter, mercury does some very useful things. It dissolves other metals in it to make instant alloys, or amalgams. A gold or silver amalgam made with mercury is an excellent material for filling tooth cavities, hardening rapidly and wearing well. It dissolves precious metals found in ores�and then can be distilled almost as easily as alcohol, boiling at only a few hundred degrees, to leave the gold or silver behind. And being extremely dense, mercury is useful for making small lab apparatus like blood-pressure gauges or the standard barometer (which would be 10 meters tall, not 0.8 meter, if it used water instead).

If only mercury were safe! But it is too hazardous to live with today, now that we have alternatives.