Hidden Cardiff: The city's tunnels and secret bunkers

A giant ventillation shaft in the Wenvoe tunnel - just one of the secret locations

For the thousands of people visiting Cardiff over this Champion's League Final weekend - the best kept secrets of the city have been under their feet.

Hidden canals wind their way under the main shopping centre in Cardiff, while beneath the fan campsite in a city centre park there lies a medieval friar's tunnel, its entrance just a couple of feet high.

While behind a "staff only" door in a city centre burger bar, opposite the castle where a dragon was guarding the Champions League trophy, lies an elaborately decorated mahogany panelled room.

Explorer Will Millard, who previously focused on more exotic locations, has discovered some of the hidden parts of Cardiff as part of a new BBC programme.

He said: "The tunnel in Bute Park was built in the medieval era where Blackfriars and Greyfriars were still operational.

"They had this secret tunnel which runs under the city and through the city centre park. It was a 2ft (0.6m) access tunnel with tightly woven stones which had been hand-placed there by friars in the past. No-one knows what it is for. It is amazing to speculate what it was used for.

"After 25m (82ft) it collapses and it is hard to continue on, but it links up to the friary in Bute Park. There have been rumours of a hoard from a bank robbery being hidden in there in the past."

The hidden entrance to the friars' tunnel

He also discovered a secret dining room located in what is now the fast food chain Burger King on Queen Street.

"I had no idea there was this opulent drinking and smoking room through the door of Burger King. They own the building but that room is a listed room so they can't do anything with it. They can't change it, and it is not really in their interests for people to know about it. But it is an incredible museum piece. It has got a church grandeur to it."

He also uncovered Cardiff's role in the UK's defence against nuclear war where he entered a huge concrete shelter through an air vent - an experience he found unnerving.

"I've walked across jungles on my own in western Africa. I've stayed with isolated tribes in New Guinea, but walking around this place up on the hill in the dark, this is definitely as scary as it gets for me," he said.

Hard to find - part of a nuclear bunker in the city

• Watch Hidden Cardiff with Will Millard, BBC One Wales, 7 June at 21:00 BST