As I recall a hole was drilled in the "door" and a fiber optic tool inserted to show what looked to me like a rock but which they called another "door". Depth was not discussed but the televison depiction would cause me to guess it was about 6 to 8 inches behind the first "door".
The only apparent justification for calling it a door was the presence of two metal (copper?) clips on it. Tjere is no doubt in my mind that there are other rooms in the GP. Where are they?
jjw