This May marks the 40th anniversary of the radical student riots of 1968, when thousands of students backed by ethnic minorities rebelled in a series of anti-government protests worldwide.

Whilst student movements led to varying degrees of political change in Europe, the US and South America, the uprising in Paris which eventually led to the collapse of the De Gaulle government is widely regarded as the most significant revolt of May '68.

The demonstrations coincided with the wider General Strike, and - at its worst - saw pitched street battles with police in the Latin Quarter. The rioting forced the creation of a military headquarters in the city to control the violence.

Most recently riots in the Northern suburbs of Paris, after two youths were killed in an accident involving a police car, led to reprisal by youths armed with air rifles and shotguns.

But whilst the rioting sporadically surfaces, an unseen youth rebellion manifests itself underneath Paris.

It’s 11pm on a Friday night, and I’m standing in an abandoned train tunnel, clad in Wellington’s, waterproof trousers, an anorak and a head torch- the light from my brow illuminating a hole in the hard clay where the tunnel wall meets solid ground.

And this hole, I know, is but one humble entrance leading to one of the largest networks of underground tunnels in the world.

The system dates back to the 12th century, when limestone was quarried to build the Louvre, Notre Dame and other notable edifices. Bones began to accumulate by the 17th century in quarry’s now known as the Catacombs

But these bone galleries, now open to the public, are just a tiny slice of the true extent of underground Paris. The intersecting tunnel network is estimated to cover 300km; these being used for military operations and police training.

With graffiti dating back to the 1700’s, the spirit of rebellion is historically evident; when the French Resistance used them as a base to sabotage the Nazi occupation in the Second World War.

And now I’m standing here waiting for the Catephiles to arrive, an infamous name referring to urban explorers who navigate the system of tunnels, tombs and bunkers.

Small pockets of light appear in the distance, with the accompanying sound of trudging boots becoming louder and louder. They move quickly, and don’t look at all surprised when they see me there waiting, uninvited, for them to descend.

"Excuse me," I say in my limited French, "I am looking for the Catacombs." They look me up and downprobing further before my assimilation into their group can be completed.

After crawling through the gap in the wall, our feet splash into the trickle of water that lines a small clay passage and we descend through a series of small tunnels that twist and turn into obscurity.

I am introduced to the guide who calls himself ‘Cosmo-Flow-Flow’. He leads the small group and they all shout lyrics to a lively, clan song, characteristic of Cataphilic groups to assert their identity- and to let everyone know they’re coming.

Cosmo-Flow-Flow estimates a rough figure of around 400 explorers in the tunnels with us, with Friday night being particularly busy.

Whilst ‘the Cataphiles’ is a catch-all term used to describe the many different, mostly young groups of teenagers who traverse the tunnels, other more notorious groups use names like the Perforating Mexicans or Les UX.

In 2004 the police discovered a bar and cinema complex under the river Seine, complete with professionally installed electricity and telephone cables. Les UX claimed responsibility, and when the police returned three days later to assess the electricity installation they found a note that simply read ‘Don’t try to find us’.

The construction of the tunnels varies wildly from area to area in terms of size and shape, as well as materials. Some are large concave, red brick tunnels whilst others are damp, and made from clay tunnels with low ceilings that seem to constantly come into contact with my head.

As the ground begins to level, and the descent seems to be over, we enter a long tunnel lined with concrete foundations. Wide gaps in the big grey pillars open up into intersecting clay passageways of varying sizes.

Cosmo-Flow-Flow points to a big opening on the left. We all stop to shine our torches into a massive hole filled with luminous, clear water that sits just beneath the tunnel floor. Though mesmerizing, the incalculable depth of the hole is frightening.

Then there’s the shouting. I can’t hear what’s being said, just the muffled, barks coming from ahead. Cosmo-Flow-Flow is nowhere to be seen, whilst a younger member of the group next to me says "Ah merde! Les Cataflics!" (Oh shit, it’s the police!).

Two plain clothed men run straight for us, one of them continuing past us, while the other stops and round’s us up. They demand identity cards, taking them from my three acquaintances.

A third police man joins us; he is in full uniform, with a powerful torch that is blinding in the darkness. I pretend to not speak any French, and say as badly as I can "Je suis Australian."

We move quickly, one policeman behind me checking every intersection, whilst another policeman in front of me snaps questions at Cosmo-Flow-Flow.

The one thing they want to know most is what group we are, and who the guide is.

Since November 2, 1955, access to the tunnels has been forbidden and now a special police task force known as the Cataflics patrol the tunnels looking for trespassers.

The police group is made up of 16 officers, and are well known amongst the Cataphiles and other groups, with some of them intercepting police radio’s to check their location as well as mapping out areas where police patrol.

We are taken to a room where, along with around 50 other Cataphiles, we are routinely searched for weapons and drugs. The police want to know if we are associated with other notorious groups, and once they realise we aren’t they become more relaxed.

We are then led up a ladder shaft, where on the surface we are made to line up and receive fines of 60 Euro. Myself along with five other Brits are let off as we do not have ID on us.

As we are released and walk through the Paris streets en masse, covered in mud and sharing anecdotes, it soon becomes clear exactly where everyone is going.

Back at the train tunnel, and slipping into the hole it occurs to me that it would take a lot more than a slap on the wrist and a 60 Euro fine to keep these enthusiasts from unlawful trespass.

It may not be the of the political kind, but this is a sure sign that the anti-authoritarian spirit of 68’ persists in the youth today.