Stonehenge is one of the most famous sites in the world. It is the remains of the standing stones set in the earth. I highly recommend watching the NOVA | Secrets of Stonehenge – PBS documentary. This has been on my list to see for years. I highly recommend that you see the documentaries of place before you visit it.
From the picture below you can see the size of the stones compared to the people. They are extremely large and heavy. The documentaries explain how Stonehenge was made. As an engineer, it is very interesting.