Figure 1. Map showing waveguides on the territory of the former USSR: (1-3) waveguides in the (1) upper, (2) middle and (3) lower crust; (4) areas where waveguides are reliably absent; (5) areas with two waveguides in the upper and middle crust; (6) areas where waveguides are probably absent; (7) areas where waveguides are probably present in (a) upper and (b) middle crust; (8-10) position of crustal waveguides as constrained by surface waves from earthquakes: (8) upper, (9) middle and (10) lower crust.