Underwater fibre optic cable, artwork. The optical fibres (red) are encased in a core tube (copper-coloured). Around this are several layers of twisted steel fibres and sheaths, culminating in the outer sheath (black and yellow). This makes underwater cables extremely strong and capable of resisting immense forces. Each optical fibre consists of a core (clear) with an outer cladding (red) with a lower refractive index than the core. Light travelling through the fibre is contained within the core because of total internal reflection at the core/cladding boundary. This allows data to be transmitted over long distances with little or no absorption of light.