Nicknamed the 'Battle Box', the armed services underground headquarters was built in the late 1930's, where Pacific Place stands today.
Here's what we know about it so far. It's not much, so if you can add anything, please leave a comment below.
Reader Malcolm Bradbury starts us off. He has kindly emailed us:
I have a book titled VICTORIA BARRACKS 1842 - 1979 published by: Headquarters British Forces, Hong Kong.
A new phase of building 1930 - 1937
The 1930's saw a new phase of building........The developement of air power meant that these buildings were too vunerable to house a war -time Headquarters. It was decided therefore that the new Headquarters building should have an associated tactical Headquarters underground. The excavation of this began in 1935 and work continued on its fitting out for some time after the official opening of the new Headquarters building in 1937. When completed it contained 36 rooms: briefing rooms and offices, a telephone exchange, signals and cypher rooms, a limited number of rest rooms and ventilating rooms..........
Next is a brief mention in an overview of Hong Kong's civil engineering by CM Guildford which says it was 'completed in 1940'. It wouldn't wait long before being put to use.
In December 1941, Major General Maltby is in command of Hong Kong's armed forces. The Battle Box is manned on 8th December, as Japan begins it's attacks against the Allies. On the 8th December, Japan begins its attacks against the Allies. At 5am that morning, 'Major General Maltby and staff descend, via an entrance immediately north of Kennedy Road near today's British Consulate, into the Battle Box fifty feet under Victoria Barracks'.  It would be Maltby's home until the surrender on the 25th December.
There are occasional mentions of the Battle Box in wartime diaries, but only as far as they were sending a message to, or receiving an order from the Battle Box. I'm interested to learn more about what was actually happening there, and what conditions were like.
Reader 'Mac' heard some tales about the Battle Box during and shortly after the Japanese occupation:
I [lived in Victoria Barracks and] asked my father about it without giving away that we actually used to go there and he was quite knowledgable on it, it was built before the second world war and was used by the japanese as a coms site untill they surrendered.
It was then used in the fifties/sixties as an emergency shelter and comms (standby)in case of nuclear war and was abandoned once hms tamar and it's systems were brought online.
There is ( or was ) a sister bunker on Stonecutters that we also visited more than once which had a direct phone /telex cable link.
The next information we have is from reader 'palmersinspain', who writes:
In the 1960's the HQ telephone switchboard was located in an underground network of rooms that were accessible via 109 steps below the HQ Land Forces building.
Personnel - Telephonists and technicians of 252 Signal Squadron worked down there 24/7 on a shift basis. Other staff included Chinese civilian telephonists. If you were on shift during a typhoon you had to stay down there until it passed as the water tumbled down a gully adjacent to the steps and overflowed making the steps inaccessible.
From this level there were other steps that led down to an IRON DOOR from which you could only exit (not enter). From this door it was a short walk to the old Victoria Barracks Sgts Mess.
Circa 65 to 68, my wife was a switchboard supervisor and I was a shift technician in this area then known as Force Exchange and Fault Control.
All communications from this area was by cable.
The other rooms in the complex were only opened during security alerts when they were manned by key personnel from the HQ.
Then another snippet from Malcolm Bradbury. The book he mentions above includes several photos of the underground HQ, one of which has the caption "The underground headquarters have not been used since the internal security operations of 1967, when it was found necessary to form combined operations rooms with the Hong Kong Police."
By the 1970s, it appears the complex has been abandonded. Reader 'Mac' again:
I lived in victoria barracks in the mid 70's till the 80's and i can tell you exactly what is behind that door i used to go there with friends to explore and be scared by massive black spiders ( we were young teenagers then).
As you enter the complex through the rusty door there is a passageway to the left which goes alot deeper into the bunker and there is a short passage to the right that only has a 'fire hole' in the wall behind wich was a small guard room. This was to allow soldiers to visually vett anyone entering and if needed open fire on intruders from a 'pillbox' opening.
The passageway goes deeper into the hillside and the first corridor feeds back on itself and leads to the guard room and at the same place there is a sand pit that was used to check that any firearms were not loaded on entry.( the duty seargent would take the rifle/pistol and 'fire' it into the sand ensuring the chamber was empty)
After this the passageway ( there is only one ) has three or four ( it's getting to be a long time ago!!) rooms, one of which was circular. They were mainly used for communications and when we were there there was a sheet of glass standing upright in the centre with an outline map of HK island, stonecutters and kowloon.
We found loads of other bits such as pens, cups, empty cartridge cases, documents, maps and so on which we tended to play with or take home. But we were good lads really and did not break the map, i wonder if it's still there or is hanging in some chineese officials office now...............
The other rooms were a mess and small kitchen, a briefing or ready room with long tables and BIG telephone cables sticking out of the wall and a store room that was bare ( as i remember ) except for racking and shelving.
It next appears in a report commissioned by the government, and published in October 1983. From the introduction, it is clear that the complex was not widely known about in Hong Kong:
During the setting out for the proposed access road to the Queensway New Supreme Court in Victoria Barracks a number of ventilation shafts were discovered on the road's alignment. It was known that the disused Combined Services Underground Operating Headquarters lay beneath the Barracks in the area, but no detail of the exact position or extent of the Headquarters could be found. 
The report goes on to say that, once they found out where it was and how to get in, it was in good condition apart from some blackening and charring from a fire in one room in early 1983.
However, with the old Barracks area slated for re-development, the complex didn't last much longer. Its final destruction was described in the article Wartime network of 'streets under fire' by Guy Searls in the 6th April 1992 edition of The Standard newspaper:
When I first came to Hong Kong there was a whole string of sealed up portals along Queensway. They covered what had been the entrances to the military command headquarters from which the defence of Hongkong was directed in December 1941.
It was still in good condition - even if a bit dusty - in 1986, when it was uncovered as the hillside was ripped away to make way for the Pacific Place omplex of shops and hotels.
The Japanese did not penetrate the command bunker until after the surrender. Commercial developers managed to destroy it in relatively short time.
I think he's got his memories a bit muddled up. There was just the single exit near Queensway that 'palmersinspain' described above. But at least the date of it's destruction is confirmed.
Once again, that's all we know so far. If you were ever working there, or heard any tales about it, we'd love to hear from you.
-  'Not the slightest chance', by Tony Banham
-  'Investigation of Disused Tunnels. Preliminary report. Victoria Barrack - Disused Underground Operations Headquarters'. Document #5341, CEDD library.