If you have been anywhere near a computer or television in the past couple of days you will have noticed that seemingly half of Beirut, in Lebanon, exploded on Tuesday. The official cause of a colossal blast in the capital’s port area was a stockpile of about 3,000 tons of ammonium nitrate. But what is it and why is it so volatile?

Ammonium nitrate is a chemical that is made in industrial quantities for fertiliser and to create mining explosions. Speaking to the BBC, University College London professor Andrea Sella said: “On its own, ammonium nitrate is relatively safe to handle. The real problem is that over time it will absorb little bits of moisture and it eventually turns into an enormous rock.” That means storing it for long periods can be a problem. The ammonium nitrate in Beirut had reportedly been there for six years.

The longer it sits, the more likely it is to become contaminated by oil and other miscreants. When that happens, it can generate its own heat, potentially becoming a super-bomb at the slightest provocation. ..