Why are there giant concrete tunnels in the desert?

Basically the same idea as the classic "ether detector" of Michelson-Morley. Just a tad more sensitive. But it strikes me that if it is only laid out in a 2D grid, it could miss stuff. Has anyone thought about adding a vertical component and having the laser split into 3 orthogonal directions vs just 2?  I suppose the logistics of building the vertical component might be daunting. Of course, it doesn't have to go up ... right?  If the 3 axis LIGO is not possible for some reason I suppose you could achieve the same effect by building a second 2D LIGO some place else on our nice round planet such that the plains of the two LIGOs were roughly perpendicular.