16/03/2026
ANCIENT MULLOCK HEAPS & SINKHOLES
“150 years after the rush, the diggings are still moving beneath your feet…”
Most people think the Victorian goldfields are old, stable, and settled.
But the truth is this:
The ground in many goldfield regions is still collapsing, shifting, and hollowing out — even today.
Ancient mullock heaps, tailings piles, and back-filled shafts have been weathering, eroding, and weakening for over a century.
And the result is a quiet, invisible danger:
sinkholes
sudden drop-outs
collapsing mullock piles
ground subsidence around hidden shafts
This is one of the most misunderstood hazards in the forest — and one of the most common.
WHAT EXACTLY IS A MULLOCK HEAP?
During the 1850s–early 1900s, miners dug:
• deep vertical shafts
• long drives
• open cuts
• shallow surf-facing diggings
Every bit of waste rock and soil they removed was dumped into mullock heaps — mounds of loose, broken material thrown up quickly and without structure.
Mullock heaps were:
• never compacted
• never stabilised
• made from mixed clay, quartz, gravel, and voids
• full of timber fragments, air pockets, and rotten supports
• often dumped over old drives or hidden workings
Time has softened them…
but time has not made them safe.
WHY THEY BECOME SINKHOLES
Sinkholes form when the supporting material underneath collapses.
Common causes in the goldfields:
Collapsed underground drives
Old tunnels running beneath mullock heaps slowly cave in as:
• timber props rot
• roofs sag
• clay absorbs water and expands
• earthquakes or ground vibrations weaken voids
The collapse travels upward until the surface “punches through.”
😎 Back-filled shafts settling
Many shafts were filled with rubbish, timber, and loose spoil.
As organic material rots, the fill settles, creating deep voids.
Heavy rain
Water infiltrates mullock heaps easily.
It washes fine material out, leaving hollow channels.
Tree roots
A giant ironbark can anchor into old mullock.
When it falls in a storm, the root plate rips material out, exposing a void.
White-ant / termite activity
Termites eat the internal timber from old shafts, removing the last structural support.
WHERE THESE HAZARDS ARE MOST COMMON
Sinkholes and unstable mullock heaps are everywhere across the Central Victorian goldfields — but especially in areas with hard-rock quartz reef mining.
High-risk districts include:
• Castlemaine Diggings National Park
• Fryerstown / Vaughan / Irishtown
• Guildford / Yapeen
• Blackwood / Barry’s Reef
• Daylesford / Sailors Creek
• Creswick
• Maryborough / Timor / Redcastle
• Dunolly / Tarnagulla
• Bendigo Whipstick & Ironbark areas
These regions all contain dense networks of old shafts and drives, often only meters apart.
REAL INCIDENTS ON RECORD
Sinkholes in Victorian diggings have been publicly documented:
BENDIGO – 2020 & 2022
Multiple sinkholes opened in suburban Bendigo due to collapsing old mine workings.
Some were over 5m deep and required emergency stabilisation.
MARYBOROUGH – 2017
A massive sinkhole opened near the golf course, caused by a collapsed shaft.
It made national news.
CASTLEMAINE DIGGINGS – ongoing
Rangers routinely report new subsidence around old workings, particularly after heavy rain.
CRESWICK – 2023
A back-filled shaft collapsed near a walking track, prompting temporary closure.
These events show that the goldfields are still settling.
HOW TO RECOGNISE UNSTABLE MULLOCK HEAPS
Look for ground that shows:
Surface Indicators
• Circular depressions (“dish pans”)
• Fresh cracks in the soil
• Slumped edges of mounds
• Soft ground where it should be firm
• Unnatural round holes in the bush
• Soil sinking when stepped on
• Areas of unusually green or wet vegetation (water draining underground)
Vegetation Indicators
• Stunted or leaning trees
• Trees growing in perfect lines (over old trenches)
• Sudden patches of low vegetation in a forest of tall timber
Historical Indicators
• Rows of mullock aligned with quartz lines
• Tailings heaps shaped in conical piles
• Machinery pads nearby
• Old tramway lines
• Exposed timber fragments in soil
HOW PROSPECTORS ENCOUNTER SINKHOLES
Most encounters happen because:
• Mullock heaps ring like crazy on a detector
• They contain relics, bullets, tools, wire, and iron scraps
• They’re flat on top and easy to walk on
• They’re often the highest point for better swing angles
• Good gold was often found next to mullock heaps
But these are the exact spots where collapse risk is highest.
Prospectors have reported:
• Foot punching through soft crust
• Sudden depressions forming under weight
• Entire mullock slopes sliding downhill
• Leg-depth collapses when digging
• Hearing ground “crunch” or shift underfoot
Some collapses drop less than a metre.
Some drop several metres into old drives.
HOW TO STAY SAFE IN MULLock FIELD AREAS
DO:
• Walk around (not over) steep mullock heaps
• Stay off rounded piles with no vegetation
• Check ground firmness before stepping on depressions
• Avoid detecting at the very edge of heaps
• Step lightly around shafts or pits
• Keep dogs and kids well away from mullock slopes
• Be extra cautious after heavy rain
DO NOT:
• Dig directly on top of mullock peaks
• Stand close to the lip of depressions
• Enter holes or erosion washouts
• Assume a previously safe area is still safe
• Climb tall, steep conical heaps
• Detect inside known collapsed areas
THE REALITY
The goldfields look old…
But underground, they’re still alive.
Every mound, trench, and depression is evidence of thousands of miners shifting the earth — and nature slowly reclaiming it.
The hidden dangers are quiet, subtle, and often invisible until the moment the ground gives way.
Be smart.
Be cautious.
And remember:
If the ground looks like it was touched by miners, treat it with respect — they dug more holes than you can possibly imagine.