I recently saw this photo of the project "NATO-L", or North Atlantic Transmission One-Link. It's an under-sea powerline planned to connect Europe with North America.

Does anyone know what frequency such cables run at? I was thinking it looks like a giant coax cable, and so it must have characteristic impedance and other related parameters. I seem to recall the big over-land powerlines run at 400Hz. I searched google about this cable, but I could not find any information about the frequency.