Between 1966 and 1974, France exploded 41 atmospheric tests in French Polynesia, after which test continued underground, on the atoll of Mururoa. Mururoa is a seamount that was formed more than 7 million years ago as a result of a volcanic eruption beneath the sea. When lava hit cold water it formed intertwining tubes of rock, which built up a mountain. That mountain eroded and a basalt base with a middle layer of soil remained. The top layer of limestone and corals leaks and it is unclear if radioactive material will start surfacing. Tests showed that radioactivity was found in samples, but the levels were very low. As a result of the nuclear tests, parts of the atoll have collapsed.
Nowadays tests are banned in Mururoa, but a nuclear time bomb is likely to keep on ticking beneath the atoll. It is not advisable to visit Mururoa.