During World War I an estimated one tonne of explosives was fired for every square metre of territory on the Western front
I'm having trouble finding a reference, but I don't find 2.5 million tons unbelievable. The comparison to Vietnam is silly because that was a much smaller and different war, and aircraft have limited weight capacity.
It's said that some artillery bombardments in WWI were called drum fire because it sounded like a drum roll. During Verdun there was so much artillery that it's said you couldn't distinguish the individual explosions, it was just a load continuous roar that went on for many hours. Perhaps that battlefield is where the statistic comes from.
They were going through tens of thousands of shells a day and were quickly running out after a few weeks in. Many factories were converted or built to produce explosives. Dozens of trains were built to transport them to the front lines where they were fired continuously for 4 years. The scale and duration of the war was just insane.
I think you are misreading the OP. 2.5 million tons is the amount that would be required to cover a single square mile at 1 ton per square meter. Since the Zone Rouge is many square miles, it would require 1.2 billion tons of explosive for the 1 ton per square meter figure to be correct.
"I don't think the combatant nations produced 2.5 million tons of explosives altogether during the whole war" should be read as "While 2.5 million tons for the entire war might be plausible, 500 times that amount for a single front is clearly an error". My guess would be that someone along the way confused "acre" or "hectare" with "square meter".
http://encyclopedia.1914-1918-online.net/article/nitrate says:
> Great Britain, for example, stepped up production of powder and explosives from 50,000 tons in 1914 to over 1,860,000 tons in 1917
http://www.bbc.com/news/magazine-17011607 has a graph of the British and German explosives production during the war years. It looks like about 625,000 tons, with 285,000 by Great Britain.
I have better feelings about http://rsnr.royalsocietypublishing.org/content/68/2/171 ("Lord Justice of Appeal John Fletcher Moulton and explosives production in World War I: ‘the mathematical mind triumphant’"). Figure 3 shows Great Britain manufactured about 340,000 tons of high explosives, and 400,000 tons of propellant. (Rough eyeball numbers.) But that gives only about 350,000 tons for the year 1917, compared to the 1,860,000 tons earlier.
Table A7, p122 of https://books.google.com/books?id=2YqjfHLyyj8C&pg=PA121&lpg=... lists partial wartime production as 409,000 tons of explosives for Great Britain, and 350,000 tons for Germany, or 759,000 tons total.
Table 9 p87 of https://books.google.com/books?id=q_G_Qnag_TcC&pg=PA64&lpg=P... says Great Britain produced 409,000 tons of powder and explosives.
I can believe 2.5M tons produced by all sides during the war. Barely.
I can't believe that would all be on a single square mile. That would be equivalent, after all, to a 2.5MT nuke, or a Little Boy/Hiroshima-sized bomb equivalent every day for 3-4 months.