These subterranean, often man-made, spaces are always a bit of a mystery. That's because we'll never really find out the full story behind them now that so many years have passed.

I heard about two unusual finds only recently. But first, let's look at probably the biggest mystery of them all.

It all started on a very wet day back in Newcastle city in June 1971. The scene was near the intersection of King and Newcomen streets below the Newcastle Club.

That's when a stone wall more than a century old above King Street suddenly collapsed after heavy rain.

The collapsed section was part of the retaining wall below the club's bowling green.

Tonnes of rock and soil suddenly cascaded down onto King Street, knocking out a safety fence before spilling out, leaving an enormous pile of debris partially blocking the road.

Soon unearthed also in the side of the hill was a large mystery tunnel measuring about 12ft wide (3.6metres) and high.

Engineers and mining experts were puzzled. Was it the entry to a convict coal tunnel perhaps, or did it have something to do with once draining major stormwater off the hillside?

Old Newcastle Municipal Council records merely showed that workmen had cut a road (the future King Street) through the side of a hill there about 1866.

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Council had then built a substantial retaining wall above the excavated road. That was to prevent any slide of the building that had originally stood on the Newcastle Club site above or the Cathedral graveyard next door.

Experts examining the mystery tunnel soon came away more mystified than ever. Inside its brick arch entrance, an inspection of the tunnel revealed it only extended about a further two metres.

It then stopped at a brick wall. When part of this inner barrier was discovered, the Newcastle city engineer Mr J. Baddeley made a hole only to find a bank of white sand behind it.

A steel rod was then pushed through it to indicate this tunnel mouth had been probably totally filled with sand.

Baddeley believed the condition of the tunnel and the absence of stains on walls indicated it had not been used as a stormwater or sewer outlet.

Another site investigation, but this time by a mining geologist, concluded that it was probably a shaft of a coal mine worked by convicts about 1820.

It was at the level of the old Yard Seam where convicts mined from 1817. These mines, however, were never fully developed as those farther up the hill on the site of the old Watt Street (later James Fletcher) hospital because of water in the workings.

Mining expert Jim Comerford said the tunnel could either be of convict origin or later driven in by the private Australian Agricultural Company as a drainage tunnel to release water from uphill workings. His reasoning was that bricks were scarce in Newcastle in the convict era.

The tunnel mouth was soon sealed and the historic retaining wall rebuilt.

But there was a lighter side to the tunnel drama. It prompted a Herald cartoon by the late Les Lumsdon. It showed a well-dressed man feverishly digging inside the tunnel with a pick while two gentlemen above watch him. One then comments dryly: "Blackballed in 1927 and he's still trying to get in."

A block away further up The Hill, two surprising discoveries were once made on the old James Fletcher Mental Hospital site close to the corner of Newcomen Street.

Former site clerk of works Duncan Brown told me about them recently. In 1969, workmen had first found an unknown giant military barracks cistern from the 1840s that had been preserved. Later there was demolition work nearby before rebuilding on site was to begin.

An old Presbyterian manse had once stood on this site and there's part of an old brick wall left as a nod to the spot's history.

"People there that day with a drill rig were doing ground exploratory work. That's when I was told workers had suddenly lost a drill extension down an unmapped hole," Brown said.

"I then investigated by dropping down my tape, which was 100ft (30m) long, with one end attached to a brick. It didn't touch bottom.

"I think they might have found an early drinking well, hidden for years. Oh yes, that land is a very interesting site," Brown said.

Now, another mystery, this one also new. This time it came from Geoff Elliott, now 77 years, of Charlestown, who rang recently to tell of a mystery space discovered off Scott Street, in Newcastle's East End, probably near the old Stationmaster's Cottage

"I used to work for Royal Newcastle Hospital (RNH), initially as an apprentice electrician," he said.

"In the early 1960s I was sent as part of a work crew to repair a house there. It was almost opposite Pacific Street.

"During work we were told to move this big, heavy block of sandstone on a lawn. We did so and found what looked like a huge underground sewer system beneath," Elliott said.

"It was a brick tunnel with about three entrances and it might have run off towards the harbour. The walls seemed to be very old convict bricks. Absolutely beautiful work. We had no ladder long enough to get down into it. It may have been about four metres deep.

"I think a person may have been able to walk upright in it. But we were worried about gas and so forth. The space seemed square with an arch as far as we could see with a torch. But it's now long gone," he said.

Elliott said the mystery space might have been a convict drain, or an underground tank for the historic convict Lumber Yard/stockade (1804-1847) next door.

"I'm probably the only one left alive now from those tradesmen who discovered it. I think someone might want to know. It's part of our history," he said.

"But because of it being a possible hazard, the space was soon filled in. They emptied the ash from the old RNH boiler house into it."

The mystery chamber does not appear on major East End stormwater and sewer pipe plans in 1892. But Hunter Water has no records of there from the earlier convict era.

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