An Island Adventure

You clear the stream with ease, for it was only a couple of feet wide at its narrowest part, and you head towards the hill. You ignore the splashing sounds coming from behind you when a passenger stumbles and gets their feet wet.

You reach the bottom of the hill and look around.
It seems fairly wide, making an impenetrable wall of earth before you that stretches to your left and right, and you suspect it is located at the end of the island. No doubt if you walked along its perimeter you would come to the ocean on either side.
Your fellow survivors are digging around the base of the hill, inspecting the bushes and generally wasting their time as far as you're concerned, when one of them suddenly calls out.

"Hey!" they cry. "Look, I think there is something behind this shrubbery!"

You all rush over to join them and peer into the bushes.
They're right!
You can feel a draft coming from somewhere which is strange because it is a calm day with no breeze.
You move some of the leaves and branches aside and discover that there is a cave in the walls of this hill.

"Bloody hell," one of you says under their breath. "Should we go in?"