host was left speechless on Sunday's show. During the episode, which originally aired in 2023, viewers saw Fiona explore some of the grounds at London's Crystal Palace Park.
However, while exploring the grounds, Fiona discovered a hidden piece of history that not many people would be aware of.
As the programme unfolded, viewers learnt that nothing remains from the original glass building, which burnt down in 1936.
Although a voiceover of Fiona could be heard saying: "A hidden remnant from the original site still exists."
Speaking to one volunteer, called Jules Hussey, BBC fans learnt a team of people are working to try and restore it to its former glory.
While exploring the hidden piece of history, Jules explained to Fiona they were standing in the Crystal Palace subway.
The volunteer told the BBC star: "None of this has been touched since 1865. The actual engineering of it is so fantastic that it's holding up modern transport."
Giving more details, Fiona informed people the subway was first opened to the public in 1865 and was designed by Charles Barry Jr to have a stone vaulted ceiling.
Curious, the BBC star asked what it was used for as Jules shared there was once a train station on the other side, with the subway being used as a way for people to get between the station and Crystal Palace.
The subway has had occasional uses over the years, including being used as a World War II air raid shelter, although many don't know of it's existence. Calling it a 'well kept secret' Fiona was in disbelief.
She confessed: "I had no idea this was here! No idea whatsoever and I used to come around here as a kid. I went to school not that far from here."
Jules replied: "You never know what's under your feet do you?" Fiona agreed as she added: "You don't! I'm sure that's true for a lot of people who live in the area."
According to Crystal Palace Park Trust's website, the restoration was completed in September 2024, ahead of its 160th anniversary this year.
A statement on their site reads: "The Grade II* listed Crystal Palace Subway is a beautifully designed and crafted relic of Victorian construction, built to provide access to the Crystal Palace for first-class rail passengers."
Antiques Roadshow is available to watch on BBC iPlayer.
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