  Considerable data and analysis support the detection of one or more
supernovae at a distance of about 50 pc, ~2.6 million years ago. This is
possibly related to the extinction event around that time and is a member of a
series of explosions which formed the Local Bubble in the interstellar medium.
We build on previous work, and propagate the muon flux from supernova-initiated
cosmic rays from the surface to the depths of the ocean. We find that the
radiation dose from the muons will exceed the total present surface dose from
all sources at depths up to a kilometer and will persist for at least the
lifetime of marine megafauna. It is reasonable to hypothesize that this
increase in radiation load may have contributed to a newly documented marine
megafaunal extinction at that time.
