Afterdamp - [n]- a toxic mixture of gases (including carbon dioxide and carbon monoxide and nitrogen) after an explosion of firedamp in a mine.

There was hope, following last Friday's explosion at a coal mine in New Zealand. And there were expectations of another mining miracle. It was barely a month ago, after all, that 33 miners in Chile were safely rescued after a cave in left them trapped underground for more than eight weeks.

But today came the terrible news of a second blast from underground at the Pike River mine and an announcement by police authorities that all 29 miners still in the shattered tunnels were dead. "New Zealand is a small country -- a country where we are our brother's keeper," saidprime minister John Key. "So to lose this many brothers at once strikes an agonizing blow."

But - if we're honest about the chemical nature of coal mines - not an unexpected one. The uplifting comparisons to the Chilean rescue were unrealistic - and ultimately cruel. That was a gold-and-copper mine, damaged by an earth collapse, but fundamentally a stable structure that allowed for weeks of rescue work to take place.

Coal mines, by their very nature are unstable. To maintain safety, operators must manage them as if they were bombs just poised on the edge of detonating.

As miners drill into coal seams, the flammable gas methane - sometimes called by the old mining term "firedamp" - seeps out. The amount of methane in air has to be very carefully monitored. The gas is only combustible when it builds up to levels between 5 and 15 percent in the surrounding air. Below that range, there's not enough to ignite and above it, the mixture is too dense to be explosive (although it can be suffocating). Chemists saythat the riskiest amount of methane in the air is when it reaches 9.5 percent saturation - achieving a perilously unstable balance with naturally explosive oxygen. In that situation, it only takes one spark from a piece of machinery to ignite the gases into a literal fireball.

That would be the first explosion at Pike River. Some people believe that such a blowout clears the mine of deadly gases. But, unfortunately, that's not true. Instead, the explosion tends to peel open new seams in the coal, releasing more trapped methane. It layers the area with explosive coal dust. And in the aftermath of the fire, levels of carbon monoxide - known in old-time mining lingo as afterdamp - starts to rise.

Carbon monoxide is famously a product of incomplete combustion. If the flammable gas methane ignites in a mine, it tends to act as a fuse to highly explosive coal dust. In the resulting blast, the carbon-rich gases, dusts, and other materials burn. In the low-oxygen environment of the mine, they often burn incompletely, leaving residual carbon to attach to whatever oxygen is still jittering around underground. In last spring's disaster in the United States, at the Upper Big Branch mine in West Virginia, 29 miners died in an explosion. The blast was followed by a horrifying rise in carbon monoxide.

In fact, readings takenfrom bore holes drilled into Upper Big Branch after the explosion measured carbon monoxide (CO) levels at 14,000 parts per million. The maximum safe level, according to safety standards, is 50 parts per million. Carbon monoxide is a very bad actor. It's a poison - displacing oxygen in the bloodstream and suffocating its victims - and it's a very explosive gas.

This would be the second fiery blast - late Wednesday afternoon in New Zealand. After the earlier explosion, authorities measured rising levels of both methane and carbon monoxide in the mine shafts. They also detected smoke, suggesting that a fiery residue still burning below. They refused to let rescue workers go into such a dangerous environment and although this enraged waiting family members, that was a painfully right decision. "The blast was ... just as severe as the first blast," said a police commander.

Coal mines have ticked like waiting bombs since we first started tunneling them out, excavating earth's waiting - and dangerous - treasure trove of fossil fuels. By nature, they breed explosive and flammable gases. The deadliest mining accident in New Zealand historywas in 1896, a gas explosion in a mine on the same coal seam as Pike River that killed 65 people.

All the world's worst mining disasters have been stories of methane and coal dust explosions: In France, in 1907, 1,099 dead; in Japan, in 1914, 687 dead; China, in 1942, 1549 dead; in South Africa in 1960, 437 dead. In the year 2010, so far, six fatal mine explosions (including New Zealand) have occurred world-wide, killing nearly 250 people.

That we no longer see a thousand people die in a single explosion says a lot about how much safer mines have become over the last hundred years, about their reliance on use of ventilation, monitors, coal-dust removal techniques. But things go wrong - ventilation systems fail, maintenance gets a little careless, human beings, as ever, get overconfident - the fuse gets lit and the waiting bomb goes off.

We forget, we don't even think, about the courage that it takes to go down into a ticking mine every day, about the risks taken to provide us with our daily dose of energy. Consider this a reminder. Our best wishes, our sympathy and our deepest respect to the miners of Pike River and their families, and to the thousands of miners around the world, working somewhere underground today.