TRESPASSERS are being warned to stay out of Melbourne’s storm water drains after scores of sightings suggest a resurgence of risk-takers heading underground.

The Herald Sun spoke with three people emerging from a storm water drain in Melbourne’s northeast who said they had travelled from Sydney to explore the city’s tunnels.

media_camera The storm water drain network under the surface of Melbourne. Picture: Jason Edwards

Authorities have condemned the dangerous hobby saying flash floods put people at risk of injury and even death.

“Illegally entering drains is not simply a bit of fun, it could have potentially tragic consequences,’’ said Melbourne Water Acting General Manager, Waterways David Norman.

media_camera The storm water drain network under the surface of Melbourne. Picture: Jason Edwards

The two males and a female aged in their 20s who spoke to the Herald Sun under the condition of anonymity carried copies of official maps showing Melbourne’s storm water drains

The group said they had met at least three other people exploring underground and claimed to be part of a resurgence of the infamous Cave Clan — a guerilla tunnelling group started in the ’80s which publicises drain exploration in cities throughout Australia.

media_camera The storm water drain network under the surface of Melbourne. Picture: Jason Edwards

The Cave Clan was investigated by police in 2008 when two young graffiti artists drowned in a Sydney stormwater drain.

The incident was linked to messages on the Sydney Cave Clan’s website promoting the deadly tunnel.

Mr Norman warned trespassers found in Melbourne tunnels faced heavy fines from both Melbourne Water and Victoria Police.

media_camera The storm water drain network under the surface of Melbourne. Picture: Jason Edwards

“This activity is illegal and punishable by law, but also dangerous and potentially fatal,’’ he said.

“As the authority responsible for drainage services throughout greater Melbourne, we strongly discourage anyone from entering the underground drain system.’’

Tunnellers are known to communicate through online forums on Gumtree and Facebook.

Cryptic graffiti messages have been also been discovered throughout tunnels in Fitzroy, Abbotsford, Richmond, Hawthorn and St Kilda.

media_camera A Cave Clan sign in a storm water drain under the surface of Melbourne. Picture: Jason Edwards

The Herald Sun has seen photos of an underground chamber with a wall known as a ‘notice board’ which is used to pass on messages and directions.

The wall is also rumoured to contain secret clues to gatherings and ceremonies being held underground.

Markings on the notice board were as recent as November 2013.

Melbourne has more than 1500km of underground tunnels, which can produce sudden water surges causing floods kilometres away.

media_camera The storm water drain network under the surface of Melbourne. Picture: Jason Edwards

Some tunnels also contain dangerous substances which could result in toxic fumes.

Several drain deaths have been reported in Victoria over the past decade but none have directly been connected to the Cave Clan or drain exploration.

Two girls in Adelaide used Facebook to call for help after getting stuck in a suburban drain in 2012.