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Attaching a mobile phone to a stick, they dropped it into the void. What they filmed astonished them – a hidden stairway leading to a brick-lined vault. Inside, piled on top of each other, were the coffins. On top of one rested an archbishop’s red and gold mitre.

Two had nameplates – one for Richard Bancroft (in office from 1604 to 1610) and one for John Moore (in office, 1783-1805), whose wife, Catherine Moore, also had a coffin plate. Also identified from a coffin plate was Dean of Arches John Bettesworth (who lived from 1677 to 1751), the judge who sits at the ecclesiastical court of the Archbishop of Canterbury.

St Mary-at-Lambeth’s records have since revealed that a further three archbishops were probably buried in the vault: Frederick Cornwallis (in office from 1768 to 1783), Matthew Hutton (1757-1758) and Thomas Tenison (1695-1715). A sixth, Thomas Secker (1758-1768), had his viscera buried in a canister in the churchyard.

“It was amazing seeing the coffins,” says Patten. “We’ve come across lots of bones on this job. But we knew this was different when we saw the archbishop’s crown.”

Details of the find have been kept secret for months until Easter Day to make the vault safe ahead of the museum’s grand reopening next month. As the tomb had been undisturbed for centuries, there was a fear the roof may have been unstable.

A square manhole has now been let into the chancel floor, with a glazed panel that will offer a glimpse of the steps down to the vault. The coffins – which have been left exactly where they were discovered, undisturbed – will be out of bounds, for good reason. Most lead sarcophagi contain dry remains, but some bodies decompose into a viscous black liquid known as “coffin liquor”. Should the ancient casings crack, it will spray forth.