World War II claimed the lives of tens of millions over the better part of a decade; some estimate that as much as 2.5% of the world’s population was lost due to the conflict. The warring parties were engaged in an on-going effort to find bigger, better ways to create massive amounts of damage, and it seemed like few avenues were considered unacceptable. The most notable, terribly destructive innovation was likely the atomic bomb. But the war is filled with other military innovations, which, had they come to fruition, would have washed away thousands of buildings and lives.
In one case, that was exactly — and literally — the point. That’s because the U.S. tried to create a bomb which could summon a tsunami.
Nazi Germany surrendered to the Allies in the spring of 1945, ending the conflict in Europe — but not the war. The conflicts in the Pacific threatened to rage on, and as the summer approached, Japan showed no sign of giving up. The U.S drafted Operation Downfall, a comprehensive plan for the invasion of Japan — one which most assumed would cost hundreds of thousands of Allied servicemen their lives (and certainly result in even more Japanese deaths). Part of the reason for the enormous estimated death toll: the predictability of the Allied assault. Japan, being an archipelago, required an amphibious assault, and the allies already had captured Okinawa, one of the larger Japanese islands. The question, therefore, wasn’t one of subterfuge or tactical superiority — everyone knew where a U.S.-led invasion would start. The issue was simply whether the U.S. and its allies could muster enough force (and maintain the will needed) to take over Japan.
In 1944, according to the Telegraph, the United States was blasting away coral reefs in the Pacific in order to give its navy easier passage. An American officer noticed that these underwater explosions were causing tidal activity — namely, large, unexpected waves. The Americans saw an opportunity and, partnering with New Zealand reseafrchers, began Project Seal. They spent the next year of the war investigating the feasibility of creating an explosion (or series of explosions) large enough to create a tsunami on command. The colloquially named “tsunami bomb” would, if it worked, create a wave so large as to wash the shores of Japan clear of troops and fortifications alike, allowing the Allied naval efforts to invade mostly undisturbed.
According to the New Zealand Herald, the plan likely would have worked: “the operation found a series of 10 large offshore explosions could generate a 10 meter tsunami.” Further, a large-enough series of explosions originating “about 8 km from shore could create a giant wave capable of inundating a small city,” per the Herald. This is consistent with the damage caused by a naturally-occurring tsunami off of Japan’s coast in 1983, which also had 10 meter high waves and claimed the lives of roughly 100 people. (The tsunami originated 100 km off shore, explaining the thankfully smaller death toll.) The tsunami bomb could have been one of mankind’s most deadly creations.
Thankfully, it was never finished nor used. New Zealand’s National Business Review explained that, as the fall of 1945 approached, the technology still wasn’t very reliable. The bomb required a large number of explosives and coordination was key; in the words of the National Business Review, “even a small deviation from optimum would rob the wave of energy and the tsunami would be a ripple rather than a roar.” And even then, there are some who believe that the New Zealand-area tests were really a false show of arms, intended to fool the Soviet Union into thinking that the United States and its future Cold War allies were developing a rival to atomic firepower. (Those critics, per the National Business Review, are also in the business of hunting UFOs, so it may be fair to write that claim off as one of the groups’ many conspiracy theories.)
Regardless, by the time the Allies thought they had something potentially workable, it was no longer needed. Project Seal was shuttered shortly after the Japanese surrender and its documents were declassified in 1999.
Double bonus!: In anticipation of the casualty count from Operation Downfall, the U.S. minted 500,000 Purple Hearts (the medal given to servicepeople issued in the line of duty). Because Operation Downfall was cancelled, the Purple Hearts were never issued. Instead of throwing them out or otherwise destroying the medals, the military kept them on hand for future conflicts. As of this writing, the total number of Purple Hearts awarded since is less than the surplus made for Operation Downfall by 100,000 or so, so (with rare exception), any serviceperson who receives one today will be getting something originally made for the planned Japanese invasion.
From the Archives: The Bat Bomb: Another innovative bomb the U.S. wanted to use on Japan.
Related: A book on Operation Downfall.