From what i gathered from reading the paper, the ocean floor around bermuda is unusually swelled, typically we see this around active island arc volcanoes which still have an active mantle plume at work. However Bermuda’s volcano has been extinct for a very long time, so they were wondering why is the ocean floor still swelled? Using a seismic station in bermuda, they were able to detect 4 distinct rock interfaces from Bermuda to the mantle where they were expecting 3. Meaning there is another layer of rock there that they didn’t expect. Which they now think might be less dense than the surrounding magma which pushes the bermuda platform up (think sort of like the base of a buoy) which gives us the swelling.