We send roughly 1.6 billion tonnes of municipal waste to landfill every year, and for most people that is where attention stops. But a modern landfill is not a giant compost heap where rubbish quietly rots away. It is an engineered, anaerobic, surprisingly hostile environment, and what happens inside it shapes the climate, local water supplies, and the land itself for generations.
A sealed box, not a compost heap
A properly engineered “sanitary” landfill is designed to keep waste isolated from the environment, not to break it down. The base is lined with thick clay and plastic membranes to stop liquids escaping. Waste is dumped in cells, compacted by heavy machinery, and covered with soil each day. When a cell is full, it is capped and sealed.
The goal is containment, and it has a strange side effect: because the contents are packed tight and cut off from oxygen and sunlight, decomposition slows to a crawl. Researchers excavating old landfills have pulled out decades-old newspapers still legible and recognisable food (heads of lettuce, hot dogs, carrots) that had barely degraded. The landfill mummifies as much as it digests.
The methane problem
What does break down does so anaerobically, without oxygen, and that process produces landfill gas, roughly half methane and half carbon dioxide. Methane is a potent greenhouse gas, trapping far more heat than CO₂ over the short term, and decomposing waste is one of the largest human sources of it, as covered in our methane counter notes.
This is why organic waste in landfill is such a problem. Food scraps and garden waste that would compost harmlessly in the open instead generate methane underground. Modern sites install networks of wells to capture the gas: burning it off (flaring) or, better, using it to generate electricity. But capture is never complete; a significant share leaks out regardless, and many older or informal sites capture nothing at all.
Leachate: the toxic broth
Rain percolating through a landfill picks up a cocktail of dissolved chemicals on the way down: heavy metals, ammonia, solvents, and increasingly “forever chemicals” (PFAS) from packaging and products. This contaminated liquid is called leachate, and it is one of the most dangerous things a landfill produces.
The liners and drainage systems exist to collect leachate so it can be pumped out and treated. But liners can tear, degrade, or be overwhelmed, and once leachate reaches groundwater it can contaminate drinking-water aquifers for decades. Managing leachate is a cost that does not end when the landfill closes; it can require monitoring and pumping for thirty years or more after the last truck leaves.
Why “biodegradable” barely helps here
Many products marketed as biodegradable or compostable need oxygen, moisture, microbes, and often industrial heat to break down. A landfill provides almost none of those. A compostable cup buried in a sealed cell may persist nearly as long as a plastic one, and if it does degrade, it may simply add to the methane problem. This is one of the central confusions explored in our piece on the biodegradable plastic myth.
Life after closure
A capped landfill does not simply disappear. Sites are monitored for gas and leachate for decades, and the ground continues to settle as the contents slowly compress and decompose, which limits what can be built on top. Some closed landfills become parks, solar farms, or golf courses; many become a permanent maintenance liability.
There is growing interest in landfill mining (digging up old sites to recover metals, recover land, and burn or recycle the rest), but it is expensive and reintroduces buried toxins to the surface. The cleaner answer remains the one at the top of the waste hierarchy: keep material out of the ground in the first place. Composting organics, recycling honestly, and above all producing less are what shrink the problem. A landfill is not where waste goes to vanish; it is where we agree to store it, and to keep paying for it, long after we have forgotten what we threw away.