Thunderbird by Jack McDevitt
THUNDERBIRD is a startling and majestic epic, returning to the world of Ancient Shores, from Jack McDevitt, the Nebula Award-winning author of the Alex Benedict and the Priscilla Hutchins novels.
A ten-thousand-year-old stargate has been discovered on a Sioux reservation. Travel through the gate leads to three mysterious destinations: a seemingly empty garden world; a strange maze of underground passageways; and a space station with a view of a galaxy that appears to be the Milky Way.
The race to explore and claim the stargate quickly escalates. Two opposing camps emerge, divided over whether the stargate is an unprecedented opportunity for scientific research or a disastrous threat to national - if not planetary - security.
Especially since travel through the stargate isn't necessarily only one way...