The coliseum is a beautiful building that is at the top of everybody's list of things to see when in Rome. And from our experience it is definitely worth it! It is amazing to walk through such a historic building knowing the history that has happened there. It is a busy site and you will have to wait in line to get in. If you spend much time in Italy, you will soon realize that there are many tour groups which offer a fast pass to get in. Taking one of these tours will get you into an expedited line and will have you touring the coliseum that much faster. Normally my wife and I like to avoid tours as they prevent us from spending the time we would like where we want to. We therefore skipped the tour and waited in line. Once inside we started wandering the coliseum and I was excited to make my way down to the underbelly where the true magic of the coliseum took place. While looking into the arena we could see groups touring below but I couldn't figure out how to get there. We wandered for awhile before making our way out.
Here's the tip. If you want to see the mechanism below the arena floor, you need to take a tour. We discovered this too late as the tour often needs to be booked days in advance. Therefore, if seeing the bowels of the coliseum is high on your list, plan ahead and book a tour!