Jack Williamson 
Stonehenge Gate [EPUB ebook] 

In a basement in New Mexico, four poker buddies and amateur adventurers who have discovered a dark mystery buried beneath the sands of the Sahara desert decide to do something about it.In the deep Sahara, they find an ancient artifact that will change their lives and the world, forever . . . a gateway between planets that links Earth to distant worlds where they discover wonders and terrors beyond their wildest imagination.