system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: In simple terms, what is fracking exactly and why is it controversial?
gpt
Conventional oil deposits consist of oil that exists as an underground pool that's under pressure.  To get it out of the ground all you need to do is to drill down into it.  Because the pool is under pressure, the oil will naturally flow up to the surface.  Once you've extracted enough oil there won't be enough pressure for it to come up on its own, but you can inject water down into it, increasing the pressure and causing more oil to come up.

What puts pressure on the oil is that there is a layer of rock sitting above it.  Dirt is very dense while oil is not.  Because oil is lighter than the surrounding dirt it naturally wants to "float" up through the dirt to the surface.  But it can't pass through the rock layer above it and so gets trapped.

Sometimes the rocks sitting above an oil deposit are semi-permeable to the oil.  This means that the oil can enter the rock, but the rock acts like a filter and traps it, preventing it from rising any higher.

The way that you extract this oil is that you drill down into the rock and then pump high pressure water into it.  This causes the rock to break apart, creating channels that flow out into it.  You then pump a solvent into those channels.  This solvent pulls the oil out of the rocks and into the channels you created.  You can then pump the oil out of those channels and to the surface.

The controversy around fracking has three components:

The first is in the solvents that are used.  The chemical composition of the solvents are considered trade secrets, so nobody knows what they are except for the companies using them and the EPA (in the US).  If the rock that's being broken apart is close to an underground aquifer that people are using for drinking water then there is a chance that the solvents can get into that.

The second is that the process of injecting high pressure water deep into the ground creates, well, underground pressure which can cause minor earthquakes.  Those earthquakes are too small to cause damage to modern buildings, but it is possible for them to damage the kind of very old brickwork buildings you see in some parts of Europe.

A significant source of opposition to fracking is more political than it is about the health effects of it.  Fracking produces relatively cheap oil, so people who don't want oil consumption to rise oppose it on that ground.  

Fracking is also a high technology industry - only a handful of US companies have the expertise to do it.  Fracking has resulted in US oil production surging to an all time high.  This, in turn, has caused the global oil market to be oversupplied.  This oversupply is largely to blame for Venezuela's current economic collapse, and is causing serious economic issues in most OPEC countries.  As a result, OPEC has put a significant amount of money into anti-fracking lobbying.