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In reading this Washington Post article titled Five myths about the atomic bomb, it's mentioned:

The decision to use nuclear weapons is usually presented as either/or: either drop the bomb or land on the beaches. But beyond simply continuing the conventional bombing and naval blockade of Japan, there were two other options recognized at the time.

The first was a demonstration of the atomic bomb prior to or instead of its military use: exploding the bomb on an uninhabited island or in the desert, in front of invited observers from Japan and other countries; or using it to blow the top off Mount Fuji, outside Tokyo. The demonstration option was rejected for practical reasons. There were only two bombs available in August 1945, and the demonstration bomb might turn out to be a dud.

(emphasis is mine)

I can't find any corroborating evidence to this through googling, but this seems crazy to me if only because the effects of bombing Fuji-san could surely be more catastrophic than bombing a conventional target--for example, if the bomb happened to catalyze an eruption, which could also catalyze major earthquakes.

So my questions are:

  1. Did the US really consider demonstrating the atomic bomb to the Japanese by "blowing the top" off of Mt. Fuji?
  2. If so, was any analysis done, by people with appropriate geological expertise, to estimate the risk this could carry to catalyze major eruptions or earthquakes?
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    Does the downvoter have any specific criticisms or suggestions on how the question could be improved? – spacetyper Sep 1 at 1:56
  • I would look into the Secretary of State at that time. If I remember right, he had a lot of weird ideas, didn't pass on FDR's reservations to Truman, and though he got cold feet later on, wanted to flatten a few things. – gktscrk Sep 1 at 6:39
  • I hope so. I hope that the US considered all options. Good planning considers a wide variety of options, including those that (like this one) would have involved less risk of the loss of human life. Pretty easy to estimate the human cost of destroying a city. Somewhat harder to estimate the cost of destroying a cultural icon. – Mark C. Wallace Sep 1 at 17:22
  • As for the second part: literally no one knew the actual power of these bombs. Von Neumann and colleagues made estimations, but there were large uncertainties about how such large explosion behaves. Even without this uncertainty, no one at that time had either data nor simulation tools to know how nukes would trigger tectonic activity. – Greg Sep 2 at 2:22
  • The Compton Memo which urges the concept of a demonstration as opposed to earliest un-announced deployment makes no mention of Mt Fuji. One cannot find mention of Mt Fuji in the available Targeting Committee memos either. Could someone back in 1945 have said something along the lines of, "why don't we blow off the top of Mt Fuji?" Sure someone could have, but the serious researcher is hard pressed to find that concept in print. IMHO Mt Fuji in the article cited as a potential demonstration site appears to be journalistic hyperbole for the sake of the article, not a presentation of fact. – R Leonard Sep 7 at 17:30
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"US" and "consider" are rather broad terms. I can't find any evidence that the Manhattan Project targeting committee ever considered anything other than conventional military targets, but there were plenty of other people throwing out ideas of what should be hit.

A rather informal analysis of "blowing the top off a mountain" was done in the form of the Trinity test. The verdict was that a surface detonation of a 20-kiloton bomb would produce a crater about ten meters wide and a meter deep -- a rather unimpressive outcome.

I seriously doubt any formal analysis would have been done. Any demolitions expert could give you the answer in about five seconds: detonating explosives on the surface of a solid object is completely ineffective at causing meaningful damage to that object. In order to blow the top off a mountain with Fat Man or Little Boy, you'd need to drill a hole and lower the bomb into it.

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  • This is useful information, but doesn't answer either of my questions. – spacetyper Sep 1 at 1:59
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    @spacetyper, I added a paragraph on your first question, and the final paragraph covers your second question: it's immediately obvious that dropping the bomb on Mt. Fuji will have no significant impact, so no geological analysis would be done. – Mark Sep 1 at 2:34
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    The minutes of the second meeting of the target committee might be a nice link to add to the first paragraph. – DevSolar Sep 1 at 5:35
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    There was a lot of stuff considered and even tested during the early days of nuclear explosives that turned out not to be practical, so just because we know it wouldn't have worked doesn't mean it was never considered by the military or civilian leadership. Project Plowshare considered stupid uses for nuclear bombs into the 1970s. – jeffronicus Sep 1 at 18:24
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    @jeffronicus, the closest thing to "blowing the top off Mt. Fuji" would be the "Bowline Schooner" test of Project Plowshare: a crater-forming test of a 30-kt bomb. It produced a crater measuring 300 meters rim-to-rim inside a debris ring about 900 meters across. It required drilling a shaft 110 meters deep to produce that crater -- something I doubt the Japanese would have been in the mood to let US Army engineers do in the middle of a war. And as a point of reference, the current summit crater of Mt. Fuji is about 700 meters across, big enough to hold even 104-kt Storax Sedan crater. – Mark Sep 1 at 21:32
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A 20KT warhead is probably very much on the low end of what a volcano can do and how much damage a surface explosion it would do to a mountain is debatable. Mount St. Helens was a 24MT equivalent, from the inside, for example.

Both Hiroshima/Nagasaki drops were airbursts.

A groundburst on Fuji might have turned out to be anything but benign if it churned up a lot of irradiated earth and dropped fallout in the neighborhood. Fallout profiles with bombs depend on many factors, but higher altitude bursts result in "cleaner", less-permanent radiation. However, I also really don't know how much they knew or expected regarding fallout, so it's unclear whether considerations about it would have been a factor.

Last, but not least, you and most press coverage nowadays are looking at the atomic bombs from the point of view of late 20th century aversion (relatively speaking) of civilian casualties.

In WW2 high enemy civilian casualty rates were a feature, not a bug, and the A-bomb was truly novel only in its immediacy and apparent ease of use. Wide revulsion and mutually assured destruction would only come later. Some of the Tokyo raids caused on the same order of civilians deaths in one night.

Not sure when Western countries started avoiding, or at least claiming to avoid, civilian casualties, but even as late as the Vietnam War some consideration was given to bombing dikes which might have caused near 200K deaths.

p.s. as regards the linked article, I have a really, really, hard time taking anyone seriously who cherry picks casualty estimates as they did:

As Stanford historian Barton Bernstein has noted, the U.S. Joint War Plans Committee predicted in mid-June 1945 that the invasion of Japan, set to begin Nov. 1, would result in 193,000 U.S. casualties, including 40,000 deaths.

The invasion of Okinawa, a much more limited affair, had already incurred US casualties as follows, so quoting any 40K dead estimate is raving lunacy or flagrant intellectual dishonesty.

14,009 to 20,195 dead/ 12,520 killed in action/ 38,000 to 55,162 wounded

and at 40K-150K civilians killed as well, a good deal of it due to mainland Japanese coercion, there was considerable ethical justification in wanting to avoid repeating it on the main island.

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  • Intentionally targeting civilians is, and always has been, a war crime. What varied over time was how much collateral damage the parties involved were willing to accept to achieve their military goal, real or figleafed. Press covered civilian deaths in plentiful detail prior to the atomic bombs. One side called it "outrage", the other "propaganda". – DevSolar Sep 1 at 11:16
  • Since the Japanese army often intentionally used civilians and densely populated areas as shield, and using their shelters forcing the civilians into crossfire, I am not sure how much the US considered their responsibility to save all those civilians. – Greg Sep 2 at 2:36
  • @Greg: Convenient when you are the one war party that never had to worry about civilians in the territory held in the first place... – DevSolar Sep 2 at 9:15
  • @Greg my point wasnt that the US Army had a responsibility to protect those civilians, it is that the repugnant arithmetic of mass deaths don't obviously favor invading Japan over dropping the bombs, and that including when you limit that calculus to just Japanese civilians. Likewise the assertion by the article that Japan gave up mostly due to USSR declaring war is opinion at best, quite hard to prove. USSR had lil navy to speak of so their impact would at best be a year away. Firmer ground when he talks about leaving Hirohito alone, something done later, for anti-Communist reasons – Italian Philosophers 4 Monica Sep 2 at 17:10
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    @Philosophers 4 Monica I agree with you, on ridiculous casualty estimates (especially after Normandy, believing that Japan will be much easier...), USSR role (Navy would give no consideration to USSR, and they were the main blockers of any surrender talk in the government). My remark was about "Not sure when Western countries started avoiding, or at least claiming to avoid, civilian casualties,..". I see no much logic in considering civilian casualties against an enemy who has explicit order to execute all civilians in case of loss. – Greg Sep 3 at 14:01

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