Submarine Cable repeaters (amplifiers) used for crossing oceans. Spaced about 70km apart, costing a few hundred thousand $ each, with capacity of the order of 40Tb/s : r/cableporn
JMSE | Free Full-Text | A Cost-Effective Approach to the Risk Reduction of Cable Fault Triggered by Laying Repeaters of Fiber-Optic Submarine Cable Systems in Deep-Sea
How do undersea data cables work?
Harnessing submarine cables to save lives
CS-12M Submarine Coaxial Cable Repeater (1975) : Fujitsu Global