We have updated this article, adding extra information about the source of the blast, investigating an image reportedly showing the ammonium nitrate at the warehouse, and measuring the crater, which is approximately 150 meters long. https://twitter.com/N_Waters89/status/1290755883597803520

Firstly, this image, which I first saw posted by @Dalatrm, appears to show ammonium nitrate in sacks piled on top of each other. It was possible to match the skylights, the windows and the door markings to the warehouses at Beirut's port.

This is consistent with multiple reports that a shipment of 2,700 tons of ammonium nitrate (the same chemical which formed the primary ingredient in the Oklahoma City Bomb) had been sitting at the port since 2013, despite repeated requests for guidance. https://www.aljazeera.com/news/2020/08/officials-knew-danger-beirut-port-years-200805032416684.html

We also measured the crater visible in drone footage, putting it at about 150 meters long.