Americans generate about 226 million metric tons of garbage each year, do the math, and that comes out about 2 kg of trash per person per day.  Thankfully, about a third of that refuse we generate ends up getting recycled or composted, a portion nearly six times greater than it was 30 years ago.  Still, the majority of my garbage ends up in a landfill.  It's easy to forget about your food wrappers, paper towels, and old light bulbs the second you throw them away, and when it comes to the science of trash, there is no shortage of questions like, 'how long will this stuff take to decompose?', 'what happens to it?', and 'can our trash become dangerous?'  But first, let us start with this: 'how did we deal with all this trash before landfills existed?'  The answer?  Not very well.

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For centuries, most people disposed of their trash wherever they lived, whether that was the streets, their backyards, or even inside their own homes.  Historically, cities didn't consider garbage a problem until it affected their defenses, like in 500BC, the leaders of Athens decided they might want to dispose of their garbage a mile or so from the city, so invaders could no longer use the piles of trash to try and scale the city walls.  As recently as the late 19th century in many U.S. cities, barrels of trash were just put in public places for both people and animals to pick through.  In 1896, the Chicago city council reported that its unpaved roads were, quote, "Polluted to the last degree with trampled garbage, excreta, and other vegetable and animal refuse of the vilest description."  Eventually, people began to make the connection between disease and the squalor within their cities, and waste disposal began to change.  Which brings us to the dump.

A dump is little more than a hole in the ground, and for most of the 1900s, it's how many Americans got rid of their garbage. Sometimes sanitation workers covered the trash with a layer of dirt, but not always.  Dumps caused a number of environmental problems, many the result of liquids in the decomposing trash that made their way to the bottom of the hole, aided by gravity and rain. This foul concoction is called 'leachate', and since dumps contain no lining, any hazardous substances in the liquid, often heavy metals like mercury, chromium, nickel, and lead just ooze into the soil and in many cases, the ground water.  

In the 1970s, a U.S. government study found that 90 billion gallons of leachate was entering the groundwater supply each year thanks to shoddy dumps.  These masses of uncovered rotting garbage also released huge amounts of the greenhouse gas methane, which is 20 times more potent than carbon dioxide. 

Methane and CO₂ make up between 90 and 98% of dump gases, the rest is a combination of nitrogen, oxygen, ammonia, and hydrogen.  These gases are produced as bacteria break down all of that organic waste, and how much of each gas there is depends a lot on the type of waste, though more gas gets produced as the dumps get wetter and hotter as well.  And when the dumps of yesteryear got full, cities often burned the garbage to create more space, which released a whole other array of toxic chemicals.  Even at dumps that were capped off with a layer of soil, the methane usually killed any vegetation that tried to grow on top.  Also, methane's super explosive, maybe I forgot to mention that.  

Unless it's properly vented, methane and other landfill gases move through the ground and try to escape to any open space, including nearby buildings.  It's called 'vapor intrusion,' and if the methane finds some poorly ventilated space, like your basement, it can build up and cause fires and even explosions.  In 2000, for instance, a house in Rochester Hills, Michigan was destroyed by an explosion caused by methane buildup from a landfill that had been closed for a long time. 

It wasn't until 1976 that Congress got serious about where we put our trash.  The Resource Conservation & Recovery Act not only required new standards for landfills, but it also, for the first time, divided waste into categories: hazardous and non-hazardous.  Important distinction.  That's right, until then, there was no distinction between an old newspaper and barrels of raw sewage, or like, just a million gallons of arsenic. There, just put it in the landfill with everything else.  So for the past 30+ years, landfills in this country have had to meet stricter guidelines, like being lined, usually with compacted clay and high density polyethylene.  They also have to have leachate collection pipes, so that all of that delicious trash juice can be safely hauled off to sewage treatment plants.  Modern landfills are also designed to collect and vent methane, lessening the chances of trash hole explosions.  

Still, the downside of creating what's essentially a dry tomb is that it takes forever for the buried garbage to decompose.  By compacting the soil and keeping the oxygen out, landfills have become anaerobic environments, where that diaper you threw out today will look pretty much the same decades from now.

Seriously, archaeologists have studied landfills and found totally recognizable 15-year-old hot dogs in 20 year-old buns, not to mention newspapers that remain legible after half a century.  And while estimates vary, there's a general agreement that stuff like glass and petroleum-based plastics will take a million years to decompose, if they do at all.