The Sea Below (Unabridged‪)‬

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.