Wow! I've been a fan of the Sigma Force series for a few years now, and I just loved The 6th Extinction. This is the 10th book in the series, and it's still going strong. As with all the previous books, there's a good mix of history and real-world science. Unlike in the previous books, the history isn't as central to the plot, though.
Very often in the previous books, Sigma Force would be perusing clues found in the historical record while being chased by bad guys. In this on, they're still being chased by bad guys, but the historical part only includes a few mentions of Darwin, Admiral Byrd, and a secret Nazi Antarctic submarine base.
There are gunfights, explosions, a threat (or three) to the safety of the world, and a trip through a truly alien world. It;s great fun, and it's scary to think how much of the science is real.