The dark concrete chambers of a Second World War bunker on the outskirts of Darlington

But then an East Coast Main Line train rattles by and noise fills the cutting.

These concrete cubes are the remains of the Second World War battle headquarters bunker of Croft aerodrome, which was a handful of miles south of Darlington. In 1940-41, the bunker was dug deep into the surprisingly large spoilheap left by navvies in the early 1840s when they were dug the mainline cuttings on the approach to the Tees Viaduct.

The bunker is on a public footpath beside the mainline at Dalton-on-Tees, and it appears to have been well visited since its appearance in Memories 476 a fortnight ago.

The steps leading to the underground bunker

We asked what the structure might be – it is obviously of a different magnitude to our pillboxes – and Jon Heslop, who is compiling a book on local pillboxes, said it is a battle headquarters bunker while David Thompson pointed us to the battlehq.info website, which provides all the information you could wish to know about a battle headquarters.

Should an airfield come under attack, the bigwigs would decamp to the HQ bunker – which is about a quarter of a mile from the runway – and control operations.

Inside the rooms of the bunker

Underground, they appear to have had four main rooms to work from: one was a private telephone office, another was for messengers, in the middle was the nerve centre, and then there was an observation room with a narrow slit for a look-out and for ventilation. And there was a cubbyhole for a toilet.

The bunker was accessed by a set of brick stairs, and there was an emergency escape hatch – if you do visit, the hatch is no more, so there is a just a 6ft plummet onto the concrete below, so beware.

Today, there is just mud and darkness to see – and a collection of bottles and cans which suggest it has been a drinking den at some time.

But it is fascinating nonetheless, and really should be protected.

MIDDLETON ST GEORGE had two battle headquarters. One, near the control tower of the current Teesside Airport, was filled in in 2008, but the other, at the north-east end of the runway, still exists, its brick observation platform jutting up above the grass.

The bunker at Scorton airfield, which was home to British Spitfires and American nightfighters

SCORTON also appears to have a surviving battle headquarters bunker, and we are grateful to Susan Ramsay for details of this.

Whereas MSG and Croft were sister airfields largely catering for Canadian Lancaster bombers, Scorton was connected to RAF Catterick, and it was home to British Spitfires and American nightfighters.

It opened in October 1939, and the flat land was chosen because of its proximity to the Richmond branchline.

Immediately after the war, many of the airfield’s huts were removed for agricultural buildings. Those that remained were lost to quarrying.

But a brick-built pillbox, which has three rooms, has survived. Beside it is an open hatch with a ladder which descends into floodwater.

MANY thanks for all your correspondence about pillboxes – they are very nearly as popular as icehouses! There will definitely be more on this subject in the near future, and we’d love to hear from you if there are any more wartime relics that you can tell us about.

A final line: we couldn’t help but notice that a farm next to Scorton airfield, which must be on a slightly raised hillock, rejoices in the name of Mount Slowly.