Actually, they probably don't. The vast majority of the ocean hasn't been mapped with multibeam, civilian or military. Unless it's an area that's of strategic

Actually, they probably don't. The vast majority of the ocean hasn't been mapped with multibeam, civilian or military. Unless it's an area that's of strategic interest (i.e. mostly costal areas), it's very unlikely that the military has mapped it in detail. That having been said, getting the military to declassify more of its magnetic anomaly data for the ocean basins would be immensely helpful for understanding global tectonic evolution. They have mapped it in quite a bit of detail from airborne and marine platforms. However, an accurate and detailed map of magnetic anomalies gives a huge edge in submarine detection, so it's unlikely they'll declassify all of it anytime soon.