Mercury in the Mariana Trench
Mercury is a rare element only occurring to about 80 parts per billion in its rocks. So the earths mass at 6 x 10^24Kg would mean that there would be about 480,000 billion tonnes of it. It is much heavier than water so it would naturally diffuse to any area that was lower than the normal seabed, so say a sea trench.
Considering that the Mariana Trench is off the cost of Japan, the area that had an instances of Minimata Disease from mercury poisoning of fish and shellfish in 1956, and probably well before then, the factory involved with it starting up in 1908, I would be surprised if it didn’t.
Like all elements and compounds that are open to biomagnification it allows them to accumulate to dangerous levels. Some creatures choose to do this as a form of defence and attack. Tetraodontidae or Puffer Fish are mainly toxic, eating the same sort of thing that causes algal blooms, things like cyanobacteria that produce cyanotoxins, in large quantities and accumulating them.