There was a lot to it. The seamount was reported on one chart as an area of “discolored water” (which could mean many things) but was not on the chart they were using.
FWIW, as a surface navigator in the Navy for 24 years, I would have been using the same chart they were. It was a bottom contour chart. For many years these charts were classified because of the detail of their bottom topography and everyone assumed because of that they were the most accurate open ocean charts.
Thank you.