The Guajataca Tunnel is an interesting piece of history on the island. At one point, there was a railway system that ran around the island, mostly for the sugar cane that used to be produced here in abundance and exported out.
In the town of Quebradillas, you can find the old Guajataca Tunnel that connected the town to Isabela. Now what remains is just the tunnel and a dirt path with rock-sided walls. There's a beach below if you choose to hike down; you'll likely be some of the only ones there.