On Sep. 23, 2013, a Russian-owned, Moldovan-flagged ship departed Georgia en route to Mozambique bearing 2,750 tons of ammonium nitrate, a material used in fertilizer as well as explosives.

Why it matters: The Rhosus made an unscheduled stop in Beirut, apparently due to engine problems. The ammonium nitrate never left the port, but destroyed it nearly seven years later, along with much of the city.

How it happened: Authorities did not allow the Rhosus to travel on from Beirut, either because of its technical issues or a lack of proper documentation. Attempts by authorities and creditors to reach the ship’s owners were unsuccessful. Crew members were left stranded. So was the cargo.

In July 2014, under the headline "Crew kept hostages on a floating bomb,"ship-tracking site FleetMon reported that the ship's owner — a Russian-born Cyprus resident named Gregushkin Igor — had abandoned it with four crew members still stranded on board.

• "[P]ort authorities don’t want to be left with abandoned vessel on their hands, loaded with dangerous cargo, explosives, in fact," FleetMon noted.

The crew was eventually freed, and the ammonium nitrate was moved to Hangar 12, a storage facility at the port.

• "Owing to the risks associated with retaining the Ammonium Nitrate on board the vessel, the port authorities discharged the cargo onto the port’s warehouses. The vessel and cargo remain to date in port awaiting auctioning and/or proper disposal," lawyers representing the crew wrote in 2015.

Between 2014 and 2017, port officials wrote to Lebanese courts at least six times seeking guidance on what to do with the ammonium nitrate, the New York Times reports.

• Options included exporting it, selling it, or giving it to the Lebanese Army. None of them were taken.
• “In view of the serious danger posed by keeping this shipment in the warehouses in an inappropriate climate, we repeat our request to demand the maritime agency to re-export the materials immediately," Lebanon's customs chief, Shafik Marei, wrote in May 2016.
• "As recently as six months ago, officials inspecting the consignment warned that if it was not moved it would 'blow up all of Beirut,' according to the Guardian.

Flash forward: A smaller explosion at the port — apparently involving fireworks — caused the ammonium nitrate to ignite.

• Half the city was seriously damaged, with the governor estimating 250,000 people could be rendered homeless.
• Several port officials are under house arrest. People in the streets are demanding that the entire government fall.
• Economy Minister Raoul Nehme said "incompetence and really bad management" was to blame for the ammonium nitrate remaining insufficiently secured at the port nearly seven years after the Rhosus set sail.

Go deeper: Macron visits Beirut promising a "new political pact" for Lebanon