The bell most of the world thinks of as the sound of London has been cracked since 1859. The hourly strike you hear is not the sound of a perfect bell. It is the sound of a bell that failed within months of entering service, got turned 90 degrees, and was left with the crack in place.

The story is short and easy to follow, which is why it makes such a good tour-guide payoff. Here is what went wrong, how it was patched, and why the crack has been left alone.

What actually went wrong in 1859

The bell that cracked

This was already the second bell. The first, cast by John Warner & Sons, cracked during testing and could not be saved, so the metal was broken up and melted down for a replacement. That replacement was cast at the Whitechapel Bell Foundry in 1858, 2.5 tonnes lighter than the one it replaced.

The Great Clock started keeping time on 31 May 1859, and the Great Bell struck for the first time on 11 July. It did not last the summer. By September the bell had cracked again, after being struck by a hammer that was far too heavy.

The four silent years when a quarter bell stood in

Rather than recast for a third time, Westminster left the bell alone and handed the job to a smaller one. Big Ben remained silent for four years, while the largest of the four quarter bells struck the hours instead.

For four years, the deep note Londoners had briefly come to know went missing. The hour was marked instead by one of the smaller bells that normally play the Westminster Quarters before the Great Bell strikes.

How the fix was done

The solution came in 1863 from Sir George Airy, the Astronomer Royal, and it was not a repair in any usual sense. Nothing was welded, filled, or recast. The whole approach was to work around the damage rather than undo it. Three moves did it.

The first step was to stop hitting the cracked spot. Parliament records that Big Ben was turned 90 degrees so the hammer would strike a different part of the bell. The quarter turn brought undamaged metal under the hammer. The crack stayed where it was; the point of impact moved.

If a heavy hammer had helped crack the bell, the answer was a lighter one. Airy’s solution replaced the original hammer with a smaller version. The hammer in use today weighs around 200 kilos, striking a bell that weighs 13.7 tonnes.

The last move is the one visitors always want to see. A small square was cut into the bell at the crack to stop it spreading further. Parliament notes that both the crack and the cut are still visible today.

The principle is simple: cutting away metal at the end of a crack can interrupt the path along which it is propagating and reduce the concentration of stress there. The square did not make the bell whole again. It helped keep the existing crack from growing.

Why the crack is still there

The natural question is why, in more than a century and a half, nobody has simply replaced or repaired the bell. One reason not to interfere is that the crack is now part of the sound people recognise.

Parliament describes the Great Bell with its smaller hammer as striking a “soulful E note.” Repairing or replacing the bell could change that familiar voice.

Hugh Hunt, a Reader in Engineering Dynamics and Vibration at Cambridge, made the case for leaving it alone: “The cracked bell has a characteristic sound which we have all grown to love, so maybe best leave it alone.” A cracked bell that sounds right is worth more than a flawless one that sounds wrong.

How to see it and hear it for yourself today

The crack and the square are not visible from the street. To see them you have to go up inside the Elizabeth Tower, renamed from the Clock Tower in 2012, and stand in the belfry where the bell hangs.

Parliament runs official guided tours of the tower that climb to the bells. Be ready for the stairs: the standard tour takes you up a 334-step spiral stairwell, and the climb and descent take place without a lift. The tour itself lasts up to 90 minutes, passing the clock mechanism and the backs of the four dials on the way. Each dial measures 6.9 metres across.

Public tickets currently cost £55 for adults and £35 for children aged 11 to 17. Tickets are released three months ahead on the second Wednesday of each month. UK residents can also request a free Big Ben tour through a Member of the House of Commons, although availability is limited.

You do not need to climb anything to hear the result of Airy’s fix, though. Every time the hour strikes, the deep note carrying across Westminster is the sound of a cracked, quarter-turned bell being tapped by a deliberately lighter hammer, on a spot chosen precisely because the metal there was still whole.