Dublin police are still trying to solve the riddle of the two men who entered the city’s sewerage system last Friday – and haven’t been seen since.

Newspaper reports speculate that the pair were planning everything from a raid on a local bank, the theft of rhino horns worth $60million or a terrorist attack on the nearby Dublin Castle.

The two men were spotted entering the sewers via a manhole late last Friday night.

Their strange movements were reported by witnesses to police who are now studying CCTV footage and have mounted searches of the drains.

Potential targets include the state solicitor’s office, a branch of Allied Irish Bank and the valuable rhino horn in a safe at the Chester Beatty Library.

Police did pursue the men after they were alerted to the incident but nobody was found and officers believe they may have escaped into the river Liffey.

Reports say a map belonging to the men was discovered in one of the tunnels.

Dublin City councillor Mannix Flynn, who represents the area, has raised concern over the security surrounding the valuable rhino horns in the past.

“If there was a criminal intent, the horns may have been the target,” he told the Irish Independent.

“The biggest risk in that area is the Chester Beatty because it has rhino horns worth around €50m.

“They put them out of circulation and put them down into the ground because there was an attempt to get at them.

“I raised that risk element with them. I was concerned that we have the rarest collection and we didn’t seem to be aware that there was a high-scale risk of theft. That is the most obvious target.”

The Chester Beatty Library houses one of the world’s largest collections of carved rhinoceros horn cups.

An Irish gang has been blamed for a series of thefts of rhino horns from museums across Europe in the last year.