he Island is horrifying. Oh God! The ponies! That one is going to haunt me for a while.

It's a very minimalist book. No images. The maps are very sparse and only useful for relative positioning. It's all in the prose and descriptions. You'll need to come up with your own reason for why the PCs are on the island and why they want to go into/continue the tunnels. Money or curiosity won't cut it. Some kind of mission sent you there from the mainland.