There are difficulties with combustion engines in enclosed areas - namely a lack of oxygen. The ventilation would be a challenge. I see that the longest road tu

There are difficulties with combustion engines in enclosed areas - namely a lack of oxygen. The ventilation would be a challenge. I see that the longest road tunnel in the world is Lærdal Tunnel which is 15 miles instead of the 20ish for the Channel. It also appears to not be under water and hence ventilation is probably easier.