Why is the tunnel so big? Will they fill it up with cables eventually?,(reply>), @Tom Scott I'm surprised they don't do what I do and let the wires be in a tangled mess wrapped around each other to save space.,(reply>), @FJFD Equipment for smaller tunnels cost less and there is definitely no problem about that. The problem would be the maintenance, that´s the reason for them to be this big.<br><br>We in Prague center have a thing called "collectors" (kolektory) -&gt; they used to be built under a road as a "road for services" -&gt; electricity, gas, phone, internet, ... all in one undergound corridor big enough to allow human to walk along for standard checks or repairs if needed.,(reply>), There's one under the forest of Dean, it takes a lot of maintenance,(reply>), In transmission circuits the low current means that mutual inductance isnt much of an issue, its more to do with expansion and replacement, Replacing Buried HV lines, even in rural areas cost Tens of thousands of Pound per Mile for Replacement,(reply>), The cables have heat and Electromagnetic fields so need to be apart. there's also a cable car that runs down the middle to get tools and personel down there.