How does methylmercury enter our food?

Mercury finds its way into the food chain primarily through fish.
Pelagic (Ocean-going) fish
Mercury finds it way into seafood from naturally occurring mercury found in oceanic underwater volcanoes. Ocean-going species swim in deep oceans far from human industrial sources of mercury. Freshwater fish
Mercury enters freshwater species from natural sources as well as from anthropogenic (man-made) sources such as air pollutants (thanks to rain) deposited into rivers and lakes.

In the water, bacteria transform the mercury into methylmercury, a process called methylation. Larger, migratory species, such as shark and swordfish, absorb methylmercury from the water and ingest it when eating algae and other smaller species of fish. In the deep ocean, it would take thousands of years for anthropogenic mercury to reach the bottom sediment, be methylated and rise through the food chain.