Later published in book form.
This 1927 magazine cover looks almost exactly like the 1928 book, and both are by Paul Stahr.
Four daring Americans enter a lost valley to discover a horrifying world of living metal which plots the annihilation of mankind
This entire book can be read here
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A Lost Land, A Beautiful Witch, And A Supernatural Horror From The Ends Of Time!
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This is the 4th cover for this story I have posted, and the least sexy.