Rows of green rubber boots are lined up at the entrance to the Antwerp sewers at Suikerrui 21.
You need to put on a pair when you take a guided walking tour through the hidden network of underground waterways known as Ruien.
The two-hour visit offers a fascinating insight into these mediaeval canals that once ran through the city.
There are signs along the way to indicate the streets above your head. Tours are sometimes cancelled after heavy rain, as the sewers can become flooded.
Derek Blyth’s hidden secret of the day: Derek Blyth is the author of the bestselling “The 500 Hidden Secrets of Belgium”. He picks out one of his favourite hidden secrets for The Brussels Times every day.
