The tunnel is too small to use air as an insulator so they use cable assemblies with multiple layers of insulation. Here’s some high-voltage cable spec sheets

The tunnel is too small to use air as an insulator so they use cable assemblies with multiple layers of insulation. Here’s some high-voltage cable spec sheets that show a cross-section of the assembly for voltages above 69kV: https://www.southwire.com/wire-cable/high-voltage-underground-69kv-and-up-/c/c-high-voltage-underground https://www.southwire.com/wire-cable/high-voltage-undergroun...