An adventure in Porto 13

The twins, along with their friends Pedro, Chico, and João, went to Porto for a holiday this time.
Of course, they brought their dogs, Faial and Caracol, with them. They were incredibly lucky on the train because they met Rita and her dog, Boxie. Rita had a huge group of cousins and friends with whom she regularly organised many activities.
Together, they ended up experiencing an exciting adventure. It all began with the discovery of a tunnel. Where would it lead? Would it be dangerous to explore it? Did it actually pass under the Douro, as the legends claimed?
One night, secretly, they decided to try their luck... But someone had to stay outside, on watch... What some saw, what others guessed, was the beginning of the story.