It's a sight which hasn't been seen in nearly 20 years - the bottom of the Purfleet.
Has it sprung a leak? Or has someone left the plug out..?
Today the mystery was solved when a businessman who hopes to anchor a floating restaurant in the historic dock, in the shadow of the Custom House, said he was carrying out survey work.
'The council has taken an opportune moment to open the sluice and stop logs because we needed to see if the bottom was solid and if there was any detritus or shopping trollies,' said Roger Clarke-Payton, who hopes to bring a sailing vessel called the Merchant Adventurer to Lynn.
A West Norfolk council spokesman said: 'We have drained it out to see what might be needed for the proposed boat that's coming later this year.'
While there were no trollies today, the tide left behind a lost umbrella, a scooter, a skateboard and numerous glasses from nearby pubs.
Mr Clarke-Payton had hoped to have the Merchant Venturer in situ and her galley open by Easter. He is travelling to Lynn to meet officials later this week.
Originally a tidal quayside off the River Ouse, the Purfleet became silted up and fell into disuse by the 1990s. Metres of mud were removed as part of the North Sea Haven project to restore the waterfront, in 1999.
Tidal sluices were placed across the mouth of the dock, to ensure it held water at a constant level as the river outside rose and fell.
This afternoon it is expected to fill with water again as the tide turns in the estuary.
Comments: Our rules
We want our comments to be a lively and valuable part of our community - a place where readers can debate and engage with the most important local issues. The ability to comment on our stories is a privilege, not a right, however, and that privilege may be withdrawn if it is abused or misused.
Please report any comments that break our rules.
Read the rules here