Dr. David Throckmartin’s scientific expedition to the South Sea Islands discovers among ancient ruins a portal into Muria, an unknown underground world. After the disappearance of Throckmartin, his wife and two companions, his old friend Dr. Walter Goodwin enters Muria with a rescue party, only to confront an fantastic world filled with incredible beings, astounding scientific advances, and the worship of the most evil of all creatures, The Dweller. (Introduction by Mark Nelson)

The Metal Monster is an Abraham Merritt fantasy novel.
Dr. Goodwin is on a botanical expedition in the Himalayas. There he
meets Dick Drake, the son of one of his old science acquaintances. They are witnesses of a strange aurora-like effect, but seemingly a deliberate one. As they go out to investigate, they meet Goodwin’s old friends Martin and Ruth Ventnor, brother and sister scientists. The two are besieged by Persians as Darius III led when Alexander of Macedon conquered them more than two thousand years ago.
(Wikipedia)