Take today's main picture of an old-fashioned diorama. It depicts a headland in early colonial times with a raised Union Jack and a series of worker tents nearby. Beneath this scene, there's a drift (a coal mine) dug into the cliff with a small rock platform and a path leading to a small vessel waiting to load a coal cargo.

To my mind, this 1988-built model, although unlabelled, seems to show the flank, or northern side, of Flagstaff Hill (today's Fort Scratchley in the city's East End), around 1804 when Coal River reopened as a penal settlement.

Could the scene shown at this mystery site be the "New Discovery" walk-in/walk-out convict pit from 1804 dug by Irish felons under orders from Coal River (Newcastle) commandant Lieutenant Charles Menzies?

I'd like to believe it is. Or has the model-maker simply used his imagination, and artistic licence, to recreate a scene more than 200 years ago a little up the hill above Nobbys Beach? I'm still looking into it.

For when the original Coal River penal settlement re-started back in 1804, there was nothing on site. There was no Foreshore Drive below present Fort Scratchley, no Nobbys carparks, no parkland or Horseshoe Beach, not even a breakwater out to Nobbys (then called Coal Island), which the Awabakal people called Whibayganba.

Today's foreshore at Nobbys Beach was all man-made by dumping rock and fill. Back in the very early days of European settlement, water actually lapped up to the bottom of today's cliff at Fort Scratchley.

A later penal commandant Captain James Wallis (1815-1818) began a building boom at the settlement, including our first church, a jail and the start of 'Macquarie's Pier' (Nobbys breakwater) out to sea.

But it wasn't an easy task battling constant tides and coastal storms. Work then stopped in 1823 when most convicts were transferred to Port Macquarie. Eventually work resumed, but the project took 33 years. To source enough stone to build the ocean rampart, two quarries were created behind present Fort Scratchley near its entrance, off today's Parnell Place.

In fact, our landmark Nobbys breakwater actually started well inland, and is now probably buried seven metres down forever, according to a 2013 investigation. Go uphill past the present traffic roundabout behind the Nobbys surf sheds, up Nobbys Road to a point near where it and Fort Drive intersect almost directly below the WWII guns above. Then it's only a little more to the west to where today's diorama apparently features the "mystery" mine entry on the harbour-side flank of Fort Scratchley.

So, why all the interest now? It's because I recently came across today's picture of the mine scene. I hastily took it as a souvenir back in 2011 while inspecting a small, private collection near Booragul of historical Hunter landscapes. The impressive miniature models were made by an industrious, but ageing, former colliery carpenter. He was well respected in the coal mining industry and took a lot of pride in accurately depicting old Newcastle and Lake Macquarie mining scenes.

In retrospect, did he know something we didn't from his extensive mining connections? Like, was the water deep enough to accommodate a shallow draught vessel there? Was the rough wharf only very temporary? After all, his model deepens the mystery behind the most recent investigation (in 2005) into the location of the lost early convict tunnels beneath Flagstaff's Hill (Fort Scratchley).

I doubt whether any dioramas that I saw still exist, but they do raise some curious points about our earliest coal mine by the "sea shore".

That's because I also seem to recall that the late Newcastle historian Dr John Turner suspected that an early coal mine entry had been dug just off Nobbys Road to the side of the fort, in the hill behind some later timber SES buildings (long since demolished).

There's still an indent in the hillside. But if an old convict coal mine entry once existed there, it's long been crushed by the weight of solid perimeter concrete walls that were built above it in the 1880s to defend the city against a feared Russian, yes Russian, attack.

Strangely, if a long sealed-up mine entry is indeed there, it's roughly at the same spot as shown by my 2011 picture. Maybe, though, it's wishful thinking.

Complicating the puzzle is that the military added a labyrinth of 600 metres of tunnels higher up while constructing the coastal defence battery on what was once known as Collier's Point, then known as Signal Hill and Allan's Hill. With the hillside being smothered in concrete, the convict tunnels were sealed in 1885. (The army finally left in 1972, with the site then accommodating two museums.)

The search since for the lost convict mine workings has been hindered by not enough accurate information.

But there's no denying the great detective work conducted by surveyors Monteath and Powys, aided by Coffey Geosciences, on site in 2005. Here, by overlaying an 1856 tramway map over modern maps they discovered Australia's oldest coal mine on three headland locations, then drilled holes before inserting a pinhole camera into the hidden voids.

No entries were at the old SES site, although one was nearby. All spots were on Fort Drive, the road well above Nobbys Beach. Two were surprisingly high up on the ocean side, while the third was on the harbourside, off Nobbys Road.

Coal production here ceased in 1817 when a deep shaft mine opened off Watt Street, just inside the grounds of the James Fletcher Hospital.

Gionni Di Gravio, spokesman for the Coal River Working Party (CRWP), which instigated the 2005 search for the lost convict coal mine, said it "was not the end of the story". Many questions remained unresolved.

Di Gravio, also the University of Newcastle's archivist, said that, sadly, no follow-up investigation was scheduled by the City of Newcastle, even though the site was the birthplace of Australian industry with potential benefits in tourism.

He said the circumference of the fort headland had been shaved off as miners chased the coal seam embedded high up the cliff face.

But no trace now remains, even of the red/black outlines of the three historic mine entries made on Fort Drive in 2005. They have faded over time.

Because of sensitivities over the heritage site, Di Gravio said the CRWP was not allowed to paint on the cement, but had to use "wet" chalk instead, which is why the outlines hadn't lasted.

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