The Land Below Series by William Meikle

A treasure hunt into the deepest cave system in Europe takes a turn for the worst.Now rather than treasure it is survival that is at the forefront of the spelunkers' thoughts. But their attempt to escape out of the dark deep places is thwarted.Men are not at home in the depths. But there are things that are, pale terrifying things.Huge things.Things red in tooth and claw.

The adventurers from The Land Below return to the cavern under the Austrian Alps.At first it is a rescue mission, but soon Danny, Stefan, Ed and Elsa find themselves in a fight for survival. A perilous journey across an underground sea brings them to a lost island and fresh adventures, but their attempts to return to the surface only serve to make their situation worse.Now they must flee for their lives, with all the denizens of that strange island at their heels.