Beneath our feet, there's hidden heritage that offers a fascinating glimpse into Norwich's past history. STACIA BRIGGS discovers the city's subterranean treasures.

From an ordinary-looking corridor, down a flight of vertiginous spiral stairs, the medieval oak door is the only clue that you're about to enter somewhere quite breath-taking.

Hidden beneath England's most elaborate medieval city hall, the Guildhall's vaulted cellar is one of the earliest of its kind in Britain, pre-dating the building above it by at least a century.

This is where Protestant martyr Thomas Bilney spent his last days before his execution for heresy and where infamous rebel Thomas Kett waited before being led through Norwich in chains on December 7, 1549 to Norwich Castle, where he was hanged and his body left to rot.

It is where tolls for the market were kept after collection, where the most dangerous criminals in Norwich were housed until the 17th century and where early graffiti artists carved a heart within a heart and the outline of a ship into the dank walls.

The door is thought to be the same one that Kett and Bilney would have passed through on their last journey before execution. It's a story that can't fail to fire the imagination and send a chill down the spine.

More recently, the city's valuable civic regalia was bricked up in a redundant fireplace during the second world war to protect it in the event of Nazi invasion – simultaneously, a rumour was started claiming the civic treasure had been sent for safe-keeping in Wales.

Next to the cellar, a sloping entrance leads to a far more modern block of cells which were used until relatively recently – an old-style 'ghetto blaster' tape player and a sign proclaiming soup in a cup for 7p in the tiny kitchen suggest the 1980s –and bear far less esoteric graffiti than in the next-door cellar.

The Guildhall cellar is just one of the jewels in Norwich's subterranean crown.

Michael Loveday, chief executive of the Norwich Heritage Economic and Regeneration Trust (Heart), said: 'People are absolutely fascinated with the idea that something's going on underneath their feet.

'Whenever we hold Heritage Open Days where people have the chance to go underground and see the vaults and cellars and undercrofts that the city has, they are the most popular events we have.

'It's a subject that people just can't hear enough about. We think it'd be fantastic to really make something of the fact that Norwich has the UK's largest collection of undercrofts – it's something to shout about.'

There are more than 80 identified undercrofts in Norwich, although of this number, only a few are accessible or in use. Additionally, there are ancient cellars, passages and subterranean structures and experts are convinced there are yet more to be discovered.

Keen to tap into a historical resource which could prove to be as big a draw as York or Exeter's underground tours, Heart has assessed the existing city undercrofts and has recorded folklore and legends about subterranean Norwich.

Almost 70 undercrofts have been assessed by a project team in order to record their architectural structure and condition and it has been found that 28 of them could be improved to highlight them as historic features of Norwich.

Those that already offer access (regular or limited) to the public include undercrofts at Bedford Street, St Andrew's Hall, the soon-to-be opened Bridewell Museum, Norwich Cathedral, Stranger's Hall, Guildhall, Dragon Hall, Music House, Norwich Castle, Princes Street, Augustine Steward's House and Tombland.

These undercrofts are used as bars, restaurants, caf�s or exhibition rooms or can be visited by prior appointment.

Other subterranean structures which could be easily improved include locations at Gentleman's Walk, Bishop's Gate, Bridewell Alley, Botolph Street, Lower Goat Lane and the Assembly House.