The tunnel has exists, but I believe has been blocked off someway between the kitchen and the basement of the palace. I was told that the tunnel is still accessible from the Classical Kitchens, but not from the Palace.
If you have ever been on the mezzanine of the Emperor's New Study, you can see where there was an entrance to the secret passage which crossed the corridor to the mezzanine in the Maple Room. This too has been closed off. I have tried to discover whether it was blocked off at the Study side or at the former Maple Room side - its both.
There are tunnels which extend from the Alexander Palace deep into the Park. These were certainly in use until, at least, the end of the 'Cold War'. Bob and I were fortunate enough to know a wonderful man - a geologist - who worked there Miloslavovich - Bob will remember his first name.... it escapes me for the moment. Tragically he was 'killed' on the way home from his dacha at Luga. His widow still lives in the Sophia area of Pushkin. She is a French Professor.
These were wonderful and exciting days - the late 80s and early 90s - when the Alexander Palace was a top secret military establishment. I am sure many of you know, the right wing is reputed to be dangerous (radioactivity from whatever instruments etc. which were used there). Having been in the right wing, I think this was a story which was put round to terrify people from going to close - not just tourists but, particularly locals for whom the Alexander Park (apart from during the time of Nazi occupation) has been open to the public. In fact, what most people don't seem to realise, the parks were open to the public - on obtaining the requisite carnet - during the Tsarist period, when the imperial family was not in occupation.
Christine