On Page 228 of The Hollow Earth by Raymond Bernard, a conversation between Ossendowski and Houtoukatou Jelyl Djamsrap de Narabanch Kure appears, and also a description of the subterranean world by Lama Gelong, too.
800 million inhabitants are mentioned, comprising an empire ruled by one King.
"All the subterranean passages in the entire world lead to the World of Agartha. The lamas say that all the subterranean cavities in America are inhabited by this people. The inhabitants of submerged prehistoric continents (Lemuria and Atlantis) found refuge in this Subterranean World."
"... In strange vehicles, unknown above, they travel at unbelievable speed through tunnels inside the earth".
Cheers!