MRU, central corridor
The central corridor is the axis of entire MRU underground and it's dimensions are quite imposant. You need about two hours of rapid marching to walk it through. But the real time is much longer, many detours are worth exploring, inscriptions on walls worth reading and it is also sometimes necessary to overpass water pools and avoid open shafts to canalisation placed under the floor. The floor itself is fortunately diverse enough not to become boring, sometimes it is covered by sand or is it paved or decorated by stalactites and sinter beads.