There's a difference between the altitude (the height of an object or point in relation to sea level or ground level) of something and the depth below the groun

There's a difference between the altitude (the height of an object or point in relation to sea level or ground level) of something and the depth below the ground under which it travels. Deepest in this case, in relation to the Gotthard tunnel, refers to how far under the surface of the mountain it is, from the article, "The Gotthard tunnel runs 2.3 km under the mountain at its deepest point" - note that this makes no reference to altitude.