underground city was discovered in otherworldly region of cappadocia .
it is believed to consist of at least 3.5 miles -lrb- 7 km -rrb- of tunnels and rooms .
geophysicists studied a 1.5 mile -lrb- 4 km -rrb- area using seismic tomography .
study suggests underground corridors may be 371ft -lrb- 113 metres -rrb- deep .