In the deep ocean, marine snow is a continuous shower of mostly organic detritus falling from the upper layers of the water column. It is a significant means of exporting energy from the light-rich photic zone to the aphotic zone below, which is referred to as the biological pump.

In other words, cyanobacteria grow on the surface of the ocean, die, and fall into the deeper layers. This creates a particle shower, which is called “marine snow” because it looks like snow: particles slowly falling down.

It’s pretty damn cool, eh :D