Drips of water echo off its low ceiling. And what's that in the shadows? Was it a shape moving just beyond the torch beam..?

A few yards from the bustle of King Street, down a flight of rickety stairs, lies a spooky subterranean world.

Merchants who thronged King's Lynn's Guildhall in the 15th century used a tunnel to transport their wares ashore from ship's launches.

Few who use the present-day Arts Centre or Riverside Restaurant realise the former thoroughfare stretches beneath their feet.

Its flooring is on the same level as what is now Crofters – a vaulted undercroft now converted to a caf� serving frothy coffee and cakes.

'Ships would have unloaded and brought their little boats all the way up the tunnel,' said arts centre manager Nick Neal.

'There's talk there's a similar one – a twin tunnel that goes through on the other side of the Arts Centre complex.'

Both tunnels would have opened straight on to the river, offering a secure, discreet passage lit by candles.

Dark, dank and with that all-pervading smell of river mud, the Guildhall tunnel is dry these days, stopped up to keep the river that was once the lifeblood of the town's trade out.

The reassuring glow of light soon fades. Footsteps echo in the gathering gloom, as we make our way along it.

Mr Neal's torch picks out an unmistakable object, swinging as if from a gibbet.

Someone had clearly been down between us and the medieval merchants.

Someone who left a plastic skeleton behind – along with some Halloween banners.

'We did have some ghosty people down there, who wanted to do a haunted evening locally,' admitted Mr Neil.

Bottles of uncertain vintage suggest others may have used the undercroft to celebrate – or perhaps even drown their sorrows.

Press gangs once stalked the alleys that led off King Street to the swirling River Ouse where the sailing ships tied up.

Whalers sang on the quays when they sailed in from Greenland, dragging the great blowfish behind their little wooden ships, ready to be boiled for lamp oil up the river at Blubberhouse Creek.

Lynn's maritime history is well-known. Less common knowledge is the extent of the tunnels and undercrofts which once snaked beneath its cobbled streets. There are also hidden streams and rivers banished to dark culverts.

'The whole of the Tuesday Market Place is riddled with tunnels, some of which have been blocked up, some of which haven't,' said Nick Champion from the National Trust, which owns the Arts Centre complex.

After more than 500 years, the Guildhall Tunnel was almost filled in in 1945. There were plans to convert the building into a garage.

But farmer Alexander Penrose bought the building to save it for posterity. Campaigners led by Lady Fermoy raised the money to restore it, before ownership was vested in the National Trust.

It was reopened by the late Queen Elizabeth, the Queen Mother – one of its loyal supporters – for the first King's Lynn Festival, in 1952.