It’s Christmas Eve 1924 and while people are preparing for the festivities, the City of London is busy preparing an urgent notice – they plan to close St Paul’s Cathedral.

For the past few years, there had been mounting alarm at the state of the Cathedral, with joking comments in the newspapers about a serious situation. A situation that was to lead to the Public Health Department issuing an urgent notice to “The Owner or Occupier of the Structure being St. Paul’s Cathedral”, informing them that the building was in a dangerous state and ordering them to undertake urgent repairs.

As it happened, they were already undertaking repairs, as part of a long-running maintenance programme, but the repairs to the dome had been piecemeal and unsuitable, focusing more on appearance than function.

The dome itself might have to be torn down the newspapers exclaimed.

A new exhibition at St Paul’s Cathedral has opened looking at what they call the “Great Restoration of the 1920s”. A lot of the issues were brought to awareness following the failed attempt to dig a huge tunnel and station for trams next to the Cathedral in 1912, and surveys for the tramway tunnel uncovered problems with the heavy cathedral above.

In 1913, a stone fell from the wall of the nave, and emergency works were undertaken to pump concrete into the rubble-filled walls and pillars to strengthen them.

It seemed the Cathedral, long known to be a bit of an optical illusion and appears grander than the underlying structure is, was also built less out of solid stone, than columns filled with rubble and then covered in decorative fascias. The foundations weren’t much better either.

By the 1920s, Sir Chris’s design was being described as “a somewhat bold and hazardous experiment”.

The biggest problem was the dome — it was simply too heavy for the structure supporting it. The pillars holding it up had sunk by around 4 inches into the ground, and after hundreds of years, the strain was showing.

Cracks were spreading around the dome which was looking less dome-like as each decade passed. There were concerns the dome might collapse.

The new exhibition, in the Cathedral’s crypt, is an exploration of that Great Restoration, and how it responded to the dangerous buildings notice that nearly forced the Cathedral to close entirely.

A series of display boards examine the works that were undertaken to the way it was funded. Although restoration works were already taking place, as with most things in life, it takes a crisis to mobilise people — and here was the perfect crisis for the great and good to get behind.

The King gave £100 and had his name grandly illuminated in a book that’s on display in the exhibition. The Bishop of London gave £100 and had his name less grandly illuminated on another page. The Freemasons gave £500 and got a brief mention of thanks.

Some architects drawings show how badly distorted the building had become over the centuries, and cartoons from contemporary newspapers show how they highlighted the failing state of the building.

Eventually, it was decided to reinforce the huge columns that hold up the dome, while replacing Sir Christopher Wren’s mighty iron girdle which was rusting to nothingness. A massive steel chain encased in concrete added further down the dome is now holding the dome in its more familiar circular shape.

The exhibition is a look back into the past when people climbed the outside of tall buildings with ropes while wearing tweed jackets, of large moustaches and cloth caps.

It was a time when urgency replaced slow activity and 20 years before Winston Churchill’s famous cry to save the Cathedral at all costs, the nation also came together to save the Cathedral.

The exhibition is open now in the crypt of the Cathedral and is included in the entry charge. Note, if you Gift Aid your ticket, it qualifies you for unlimited repeat visits for a year.

At least we know the building is safe and secure for the future.

Isn’t it?