I recently watched a documentary about nuclear testing in the South Pacific, and came across what seemed to be evidence of many kilotons-worth of unguarded and rather accessible plutonium-239 in 'fist-sized-chunks' under only a foot of concrete. This seems unrealistic, and I was interested to see if it made sense from the known history.
Although not as well known as neighbouring Bikini Atoll, Enewetak Atoll was the site of 31.8 megatons of nuclear testing between 1948 and 1958, amounting to 6% of the total test yield worldwide. Several of the tests failed, and one failed catastrophically with zero fission yield, throwing 400 'fist-sized' chunks of plutonium-239 over the island of Runit and into the surrounding waters. The metal chunks were collected by hand into plastic bags, and the surrounding topsoil was pushed into the test crater. The plastic bags were placed on this dome, and the dome was covered in concrete. The site had been abandoned and unguarded since 1958. It is accessible by boat, and you can see several 'explorer' types on youtube wandering around on it.
Pu-239 has a half-life of 24,110 years. If the above is true, then this is still quite certainly there in fist-sized chunks under less than 40cm of concrete. Is this really the case? Should I go get a jackhammer and become a geopolitical powerhouse?
Here is the relevant point of the documentary: https://youtu.be/autMHvj3exA?t=1694
I haven't watched the documentary, but if it says as you say it does, then it is wrong. Cactus crater/Runit dome is full of mostly dirt. It is contaminated dirt, but dirt nonetheless. The DOE estimated that it had about 545 GBq (~13 Ci) of material in it at the time it was sealed. If we assumed that was all Pu-239 (it is not), that is about 6 grams of material (because it is also Pu-240 and Am-241, both of which are more radioactive than Pu-239, then it is even less material) contaminating the dirt.
The idea that one test would contain 400 "fist-sized" chunks of Pu-239 is by itself not plausible. The amount of plutonium in a weapon, even a large one of the 1950s, was always a lot smaller than that. (The primary is going to contain like one fist-sized chunk. The worst secondary in the world would not contain more than 10 or so. Obviously a fist size can vary but 5 kg of Pu would be approximately fist-sized; the DOE would not have buried 2 tons of Pu like that (they may be negligent but they aren't stupid — that would be enough Pu to be worth separating out from any dirt it was stuck in). The US has about 88 tons of plutonium total by comparison.