> a 20 million megaton bomb (which is to say, a bomb with the yield equivalent to 200 teratons of TNT, or a bomb 2 million times more powerful than the Tsar Bomba’s full yield). If we assumed that such a weapon had even a fantastically efficient yield-to-weight ratio like 50 kt/kg, that’s still a device that would weigh around a billion metric tons

Twenty times the total amount of deuterium currently present in the world's oceans.

Basically, you'd have to completely change the chemistry of the oceans on a global scale.

Also, AFAIK there is no realistic way to synthesize deuterium artificially (please let me know if I'm wrong!) so to get started you'd need to go out and find something like an order of magnitude more deuterium than exists on planet Earth. You'd have to scour a big chunk of the solar system.

So, not something an apocalyptic cult is going to pull off in the forseable future.

Well, based on another comment in this thread [0], after the ocean's concentration of nuclear fuel is covered, maybe about 2 million Tsar Bomba's would do the trick.

The physical dimensions of the Tsar Bomba are on the wikipedia page:

https://en.wikipedia.org/wiki/Tsar_Bomba

If 2 million are needed, then that's 100 cubed, twice. So two cubes of this bomb, with each cube measuring 100 units per edge, filled solid. The bomb is already designed, so producing 2 million of them would be a matter of setting up an assembly line, and churnning them out like automobiles, once the raw materials are procured.

Each one is about the size of a car, so a parking garage that holds 2 million cars is an okay frame of reference. Most stadiums have parking lots that hold 10,000 spaces, so 100 stadiums, across one hundred cities, but all together in one place. Realistically, though, two million cars easily fit inside any large city.

As for the gaseous concentration of fusion gas in the ocean, that's a much tougher problem to tackle. Really, because the huge fusion bombs that use deuterium (and tritium) are sourcing that material from the oceans (or other bodies of water). Culling deuterium from natural water sources is among the easier ways to produce weaponizable amounts of the gas. So, in order to create twenty times the natural standing concentration would require synthesizing your own new, man-made deuterium from the normal variety of hydrogen, by irradiating non-heavy water.

For every million gallons of ocean water, you can pull out maybe 156 gallons of heavy water, so you'd get about six hundred trillion gallons of heavy water, if you could capture all of the natural heavy water in the world. Take 600 trillion gallons, and multiply it by twenty. Bingo, you're there. That's the ignitable concentration, if you had 2 million Tsar Bombas.

[0] https://news.ycombinator.com/item?id=18011959