They Were Hollow Inside
The wall was solid concrete — except for where the circles were! With this new discovery, it was becoming clear that these strange marks had been plugged up and then painted over. But why? He tapped in the middle of the circle with his fist and clearly heard something behind. It was an echo! So, whatever was hidden inside had to be something far more intriguing than an old pipe. He knew what he had to do. He had to get help!