Alien Safari: Underworld. Alien Safari, #6

Reach the dizzying heights of adventure . kilometers below. When an old acquaintance arrives seeking refuge on Hesperidia, Vaughn and Jan reluctantly give him sanctuary. Trouble follows wherever he goes, but he's also custodian of an extraordinary alien secret that he's used to save countless lives across the colonies. This time, however, his own nine lives may have run out. Diurnal, the most feared hit squad in the galaxy, has tracked him all the way to the Hesp.
And once that posse has the scent of its prey, it never leaves empty-handed .Vaughn's personal grudge against one of the assassins threatens to combust an already heated manhunt. Jan realizes their only chance for survival is to hide somewhere no one can find them: the largest subterranean cave network on the planet, recently discovered. A tense hide-and-seek soon becomes a deadly chase into the underworld, where Vaughn, Jan, Stopper and their companions must use their combined skills to outwit a foe that possesses almost supernatural tenacity.
In that mostly unexplored realm, unimaginable life-forms have flourished where no sunlight can reach. And as the pursuit delves ever deeper, the mysteries of this subterranean ecosystem provide shocking and wondrous discoveries in equal measure. The new threat from without meets ancient ones from within as Hesperidia witnesses the ultimate contest between technological cunning and the awesome potential of alien nature unleashed.
Buckle up for a breathless thrill-ride as Alien Safari ventures into timeless depths of the unknown.