Methylmercury is one of a few known developmental neurotoxin to humans. Globally, humans are exposed to methylmercury primarily via the consumption of seafood, yet the source of methylmercury in the ocean remains a subject of debate. A subsurface seawater enrichment of methylmercury was first reported in the early 1990s, but it did not catch much attention until the late 2000s. Since then methylmercury enrichment has been observed in subsurface seawater in almost all ocean basins that have been studied. Most of the studies have attributed the subsurface methylmercury to in situ mercury methylation in the water column associated with organic matter remineralization, yet how this methylation process operates remains a mystery. Based on recent studies in the Arctic Ocean, here we show that ex situ methylmercury production, especially from organic-rich shelf sediment followed by offshore transport, cannot be ruled out. This is supported by three lines of evidence: 1) the spatial distribution pattern of subsurface methylmercury in relationship with distinctive water masses; 2) a lack of known mercury methylators in the highly oxygenated subsurface even considering anoxic microenvironments; and 3) the problems associated with existing mercury methylation and demethylation rates that were determined using a seawater incubation approach. Our analysis calls for further studies on the origin and dynamics of methylmercury in the global ocean, which is critical to our ability of projecting future trajectories in human exposure to methylmercury under the Minamata Convention on Mercury in a changing climate.