Undersea cables typically have conductors in them to power erbium-doped fiber amplifiers using high voltage DC. Over long distances, solar flares will induce significant currents into these conductors that will likely damage this equipment. Every few years the IEEE runs a report on the risks to the power grid and how grid operators should really adjust their designs to incorporate fixes for this known risk. It's pretty much inevitable that humans will wait until this failure occurs before implementing design fixes.
