According to new measurements, there likely is a large hidden chamber in the Great Pyramid (Paper). There is already speculation about its contents, with people looking at religious sources the chamber may be based on. What bothers me about this is that I was under the impression that the interior decoration was done without suspending the rest of the building work, using using oil lamps as evidenced by soot. To achieve this, they would need access tunnels which connect all the chambers.
The newly discovered chamber would either need to have some other way of access or couldn't be decorated, making a religious purpose unlikely which leads me to think it might simply be stress relief for the chambers beneath it.
Are there any known chambers that are completely sealed off and do not have any external access?