1) A ship catches fire:
Back on August 13th the bulk carrier Cheshire went adrift after a fire broke out in one of its five cargo holds, The vessel as a whole is thought to be carrying some 40,000 tons of ammonium nitrate fertilizer. Keep that amount in mind. The IRA pioneered the use of fertilizer bombs but, as CBS News observed, the most spectacular bombings worldwide use this stuff. To turn it into an effective bomb for terrorist purposes you have to "grind it down" – a slow process. But a "big" fertilizer bomb would come in at about 3,000lb. That's about 1.5 tons, give or take a bootload. The Oklahoma City bombing of 1995 used a truck loaded with 4,800lb, a couple of tons, of ammonium nitrate.
So, currently there's a fire on board a ship, possibly now in more than one hold, with 20,000 as much ammonium nitrate fertilizer as was used in the Oklahoma bombing of 1995. Let's suppose it's only 1% as effective because it hasn't been "prepared" (I reckon this is a highly optimistic assumption, by the way) it's still 200 times as big as the Oklahoma City bomb
2) The Spaniards fight back
Now, the Spanish authorities have put out a few releases stating that things are "under control", but that's about the most optimistic scenario. They can't put out the fire (it might even be getting hotter). Clearly, given the potential lethality of any exposure ("the largest non-nuclear explosion ever" was how one insider called it) to a blast, you either need to be very brave or very stupid to go anywhere near it. So, basically, we have no idea how much water is being poured on this vessel, and where from. Spain said that tugs were "cooling the vessel from a safe distance", but recent photographs are remarkably thin on the sea.
The AIS (which is how people like us can confirm where it is) has been off since August 15th. So, we *think* it is drifting away from land, but we don't really know for sure. What we do know is that this is a problem with no easy solution. As soon as the fire broke out Las Palmas port responded to a request for the vessel to be brought to land so that the fire could be put out with a curt "fuck off". Basically you can't let this ticking time bomb anywhere near land. And if it isn't near land, it's not easy to fight a fire on a large ship that's drifting and un approachable because of the heat and the danger.
3) What might happen
There is, believe it or not, a historical precedent for this – after a fashion.
The Texas City disaster was an industrial accident that occurred April 16, 1947, in the Port of Texas City. It was the deadliest industrial accident in U.S. history, and one of the largest non-nuclear explosions. Originating with a mid-morning fire on board the French-registered vessel SS Grandcamp (docked in the port), her cargo of approximately 2,100 metric tons of ammonium nitrate detonated, with the initial blast and subsequent chain-reaction of further fires and explosions in other ships and nearby oil-storage facilities. It killed at least 581 people, including all but one member of the Texas City fire department.
The fire attracted spectators along the shoreline, who believed they were at a safe distance. Eventually, the steam pressure inside the ship blew the hatches open, and yellow-orange smoke billowed out. This color is typical for nitrogen dioxide fumes. The unusual colour of the smoke attracted more spectators. Spectators also noted that the water around the docked ship was boiling from the heat, and the splashing water touching the hull was being vaporized into steam. The cargo hold and deck began to bulge as the pressure of the steam increased inside.
At 09:12 the ammonium nitrate reached an explosive threshold from the combination of heat and pressure. The vessel then detonated, causing great destruction and damage throughout the port. The explosion sent a 15-foot wave that was detectable nearly 100 miles off the Texas shoreline. The blast flattened nearly 1,000 buildings on land. The Grandcamp explosion destroyed the Monsanto Chemical Company plant and resulted in ignition of refineries and chemical tanks on the waterfront. Falling bales of burning twine from the ship's cargo added to the damage while the Grandcamp's anchor was hurled across the city. Two sightseeing airplanes flying nearby had their wings shorn off. 10 miles away, people in Galveston were forced to their knees. People felt the shock 250 miles away in Louisiana. The explosion blew almost 6,350 US tons (5,760mt) of the ship's steel into the air, some at supersonic speed. Witnesses compared the scene to the fairly recent images of the 1943 Air Raid on Bari and the much larger devastation at Nagasaki.
Legal actions continued for a decade.
Should the Cheshire blow up at a distance of 150nm from land, we can hope that it will be a spectacular mushroom cloud and nothing more, but there's still 20 times as much on board as caused the Texas City disaster. It could cause a tsunami. So we just have to hope that it doesn't start drifting back towards shore and that it does't explode. 'Cos if it does I reckon it might be a big story indeed.