Le Passe-muraille

Walk into it. That is the whole instruction, and most people passing through the Pescatore end of the park never work it out.

It is two bronze hemispheres, three metres high and six across, set down on the grass with a gap between them. From outside it reads as a solid dark mass. Step through the opening and it stops being solid: eight thousand holes have been drilled through the metal, and the park comes through them as eight thousand points of light. The sculptor's other name for it is Chambre sensorielle — the sensory room — which tells you he was making a place to stand rather than an object to look at.

It was made in 2006 by Jean-Bernard Métais, a French sculptor, and unveiled on Christmas Eve of that year, with the designer Andrée Putman and the mayor Paul Helminger among the people standing in the cold. It carries no signature anywhere on it. That was deliberate.

There is one more thing under your feet. The city put this on top of the old casemates — the tunnel network cut through the rock beneath the fortress — so a hollow bronze chamber you can walk inside is sitting directly above hollow stone chambers that soldiers once walked inside. Whether Métais intended the echo or the city simply had the space, nobody says.