Secret Room Was Hiding in Great Pyramid All Along

What's inside the pyramids? When people think of the pyramids, they usually imagine mummies, treasures and golden sarcophagi. But they were much more than just that! For example, the Great Pyramid of Giza is about 450 feet tall and it's the biggest pyramid in Egypt. So it must have a lot of cool stuff inside, right? So what did the scientists manage to discover?