Why Ballarat Has So Many Sinkholes

Why Sinkholes Keep Opening in This Australian City

It’s a quiet morning in Ballarat, Victoria – until the ground suddenly gives way beneath a boy walking to school. One moment he’s on the footpath, the next he’s waist-deep in a hole that wasn’t there seconds before. Thankfully, he’s unhurt, but the incident is alarming. Not long after, a local homeowner returns from an errand to a bizarre sight: his backyard fishpond has vanished into a muddy crater, pet goldfish and all. These aren’t scenes from a movie – they’re real events in modern-day Ballarat. Mysterious holes have been opening up in yards, roads, and parks around the city. What on earth is going on beneath Ballarat’s streets? To find the answer, we have to dig into the city’s subterranean secrets – relics of a gold rush that literally turned the ground beneath Ballarat into Swiss cheese.

A City Built on Gold

Ballarat was born in the frenzy of the 1850s gold rush, and for decades it was one of the richest patches of earth on the planet. Prospectors flocked here by the tens of thousands, and gold fever took hold. At first, the gold was easy pickings – nuggets and dust could be plucked from shallow creeks and surface soil. But as the surface riches dwindled, miners began chasing the gold that lay deeper underground.

By the late 1850s and 1860s, Ballarat’s fortune had shifted from placer gold (the loose nuggets in stream beds) to the gold locked inside deeply buried ancient river channels and hard rock quartz veins far below the earth. The scale of mining under Ballarat was truly staggering. Rich golden reefs crisscrossed below the city, and ancient, buried rivers from a time long gone were full of gold and were readily sought after. Hundreds of mining companies—from major syndicates to tiny partnerships—worked the goldfield at the same time. There are so many shaft markings that entire suburbs appear blotted with golden dots. And those maps are only the recorded ones. One historical review identified over 6,000 officially recorded shafts across the field, and that’s not counting the countless undocumented or short-lived shafts dug during the feverish early rush years.

*Image shows the map of recorded gold mines in Ballarat.

Deep Leads: Buried Rivers of Gold

But here’s one of the main causes of the sinkholes that continue to open up, to this day, as the rotten timber foundations that were supporting these 150+ year old shafts and adits began to fail. And that’s the ones that were chasing deep leads – ancient riverbeds laden with gold, buried deep below the surface. Millions of years before humans arrived, natural rivers flowed here, laying down rich deposits of alluvial gold in their gravel banks. Later volcanic activity covered these old river channels with thick layers of basalt lava, choking out the waterways, and leading to the development of entirely new drainage networks.

*Image depicts Deep Leads (Red) Under Ballarat that were worked during the gold rush.

To reach a deep lead, miners would sink a vertical shaft straight down through the hard basalt rock, sometimes 100 meters or more in depth, until they hit the paydirt – literally, the ancient river gravel – below. Once they hit this layer, they began tunnelling horizontally, following the path of that long-vanished river. These horizontal tunnels were essentially man-made underground riverbeds. Miners crawled through them, scooping up gold-bearing gravel and sending bucket after bucket back up to the surface. In a way, they were resurrecting a prehistoric river, chasing its golden trail beneath the earth. And damn, they absolutely smashed these ancient rivers. Ballarat has many of these ancient rivers beneath its surface and every one of them were worked. These green (red) lines show the extent of the deep leads. Miners followed the river systems working the entirety of them out. It was dangerous, wet, and miserable work, but damn did it pay well. The result of this work is a literal labyrinth of tunnels that extend for kilometres, located directly beneath the city.

*Image depicts a deep lead mine with a shaft tunnelling down to the pay dirt, followed by a horizontal drive to gather all the deposited gold interlaid with old river gravels.

It’s safe to say that discovering a deep lead was like striking an underground jackpot – one lead alone could yield tens of thousands of ounces of gold. As mentioned above, though, the work was perilous. Flooding was a constant threat (after all, these were old river channels; water lurked everywhere), so huge steam pumps were installed to keep the tunnels dry. Cave-ins were another danger – the miners dug extensive horizontal drives, often propping up the roof with timber supports as they advanced. Still, deep lead mining flourished in Ballarat. Fortunes were made in these dank tunnels. Entire hillsides got pockmarked with shaft entrances as companies followed the buried waterways under what would later become suburbs and farmland.

Ballarat’s deep leads were among the richest in the world, but when the gold was exhausted, the miners didn’t carefully remove all those timbers or fill every tunnel. They simply moved on, leaving behind empty voids propped up by aging wood in the darkness.

Quartz Reefs and Countless Shafts

The other side of Ballarat’s underground story is quartz reef mining – essentially, going after the source of the gold. While alluvial gold came from ancient rivers, the original source of that gold was the quartz veins running through the bedrock. In the latter half of the 19th century, Ballarat became dotted with hard-rock mines extracting gold-bearing quartz. These mines looked different from the alluvial leads. Instead of spreading out horizontally along gravel beds, quartz mines usually plunged straight down into the bedrock, following steeply dipping quartz veins deep into the earth.

*Image depicts a hard rock gold mine intercepting and mining a gold rich quartz lode.

Mining a quartz reef was an expensive, industrial operation. Wealthy syndicates or companies funded the sinking of deep vertical shafts – some reaching well over 300 meters (1,000 feet) underground – with networks of horizontal levels branching out at various depths to intercept the quartz lodes. At each level, miners would drive tunnels into the rock, stope out (cut out) the quartz vein, and haul the gold-bearing rock to the surface to be crushed in massive stamping batteries. This kind of mining transformed Ballarat from a tent city of transient diggers into a booming town of engineering marvels. By the 1860s, impressive engine houses and chimney stacks rose above many mine sites, and the clang of machinery was constant.

The scale of quartz mining under Ballarat was staggering. Rich reefs crisscrossed below, and multiple companies often worked different sections of the same reef, each staking out their claim with a shaft and tunnels. Along some lines of lode, shafts were spaced just a few hundred meters apart, or even closer – imagine a line of vertical elevators descending into the earth at regular intervals, each with teams of miners tunnelling toward one another from opposite sides of a claim boundary. Virtually every hill and gully in and around Ballarat East, Golden Point, Black Hill, and nearby suburbs saw some kind of mining

During the peak of the boom, dozens of mine shafts would be active at once across the Ballarat landscape. For instance, the famous Band of Hope and Albion Consols Company (an amalgamation of two big claims) reportedly sunk 11 major shafts and produced over 22 tons of gold from a mix of alluvial leads and quartz reefs. And that was just one company – there were many others. Each had shafts, tunnels, and hundreds of workers underground. Ballarat’s very foundations were riddled with holes – some shallow, some literally hundreds of meters deep!

Abandoned Tunnels and Rotting Timbers

Like all booms, Ballarat’s gold frenzy eventually cooled. By the early 1900s, most of the major mines had closed – the easily won gold was gone, and extraction was getting too costly. The last of the big mines shut its doors in 1916, leaving Ballarat to transition into a quieter regional city. However, closing a mine in the 19th century didn’t mean erasing it. In many cases, when a mine was played out (meaning it was no longer profitable), the operators simply boarded it up and walked away. Mine shafts were usually “capped” to prevent accidents – but the methods varied. For shallow diggings, closing up might be as simple as refilling the shaft with waste rock and soil. For deeper shafts, miners sometimes threw debris down them or placed timber and planks over the top of the shaft, then covered that with earth. Out of sight, out of mind – at least at the time.

But those timbers were never meant to last forever. 150 years of damp, bugs, and rot can turn stout wooden beams into sponge. Some shafts that were backfilled might have settled over time, leaving an empty space near the top. Others that were covered with wooden caps have simply decayed to the point of collapse.

An old miner once noted that timber caps would eventually give way – and that’s exactly what’s happening now. After more than a century, the past is catching up with Ballarat’s present. The vast network of tunnels and shafts beneath the city has been mostly forgotten, but nature hasn’t forgotten them. Year by year, wood rots and earth shifts. Then add water to the mix: Ballarat’s climate is relatively wet, and heavy rainfall can saturate the ground, adding weight and softening the soil above these old mine voids.

When History Breaks Through the Surface

Now we come back to those strange incidents in Ballarat today – the playground sinkhole, the missing fishpond, the collapsing backyard in Brown Hill. These are all examples of history breaking through the surface. Beneath each of these spots was an old mining feature silently deteriorating until gravity took over.

Consider the sinkhole that opened near Black Hill Primary School, where young Oliver had his unexpected tumble. Geologists investigating the site discovered it was adjacent to a shaft from an 1860s gold mine known as the Crocodile mine. Over 150 years ago, Crocodile Co. sank a shaft and even conducted an open-cut excavation at Black Hill, pulling hundreds of ounces of gold from the ground. As you can see, the Crocodile Co. sunk a shaft in what is now a field adjoined to the school. The reports list it as a hard rock mine, but they operated between the Crocodile deep lead and the Pinchgut deep lead so it’s possible they were working these ancient river systems.

Ballarat East, the site of many famous gold leads and mines, has seen its share of surprises too. Our fishpond incident took place in a residential backyard in Ballarat East. The owner went out one morning and found a gaping pit about 4 meters deep and 2–3 meters wide where his beloved fishpond had been. The sides of the hole revealed moist soil and the timbers of an old shaft. Sure enough, it turned out an old mine shaft ran right under his garden – likely part of a 19th-century prospector’s claim that was never properly filled in.

Even public spaces aren’t immune. In 2022, park-goers in Ballarat’s Victoria Park were greeted by a large sinkhole opening in the grass. City crews who examined it found it was above an old tunnel –likely an adit or drive from a 19th-century mine that chased a deep lead in the park. It’s most likely related to the work put in by the United Hand in Hand and Band of Hope Co. They sunk five shafts and worked these two deep leads running right under Victoria Park. The city promptly fenced it off and eventually filled it in, but not before curious onlookers peered down to see eerie darkness and the hint of tunnel walls below.

Unearthing the Past

The holes opening in Ballarat today are like portals to the past. Each collapse pulls back the curtain on the gold-rush era, revealing the literal voids left behind by that frenzied chapter of history. It’s both fascinating and a little spooky to realize that underneath the paved streets, manicured lawns, and schoolyards of a modern city lies a forgotten network of 19th-century industry – an underground ghost town of mines. Most of the time, it slumbers silently. But now and then, with a creak of rotten timber and a slump of earth, the old tunnels remind Ballarat of their presence.

In the end, Ballarat’s sinkholes are a quirky mix of inconvenience and historical reminder. They’re the city’s past literally poking through into the present, keeping everyone on their toes. It’s a fascinating (if slightly unnerving) part of living in a former gold rush town. Ballarat struck gold long ago, and now the ground is paying back its debt, one surprise hole at a time. So don’t forget to watch your step – you never know when you might be standing on a bit of goldfields history waiting to resurface.

Here's the video we made on this on the OzGeology YouTube Channel: