Skyscrapers are one of the wonders of the modern world - shimmering towers of glass and steel standing hundreds of feet above our cities, seemingly defying gravity in dramatic feats of architecture.
From the world's tallest tower - the 163 storey Burj Khalifa in Dubai - to the iconic Empire State Building in New York, these giant monuments are marvels of engineering.
But one of the most crucial aspects of these vast towers is often forgotten - the plumbing.
It is the question that rarely gets asked: How do you get rid of poo flushed 2,700 feet up?
And the reality is not as simple as you might think.
For one thing the waste rockets down pipes at a speed of 122mph - equivalent to the average speed of most main line trains in the UK.
Drainage expert Mark Briggs told Mirror Online: "The basic thing to remember is the poo hits terminal velocity - just like a sky diver.
"So after about five floors up, in theory you can go as high as you like.
"It does bounce back up at the bottom though so the drainage needs to be designed to stop that."
In order to minimise bounceback of waste from pipes running down many storeys of buildings, the bottom of the 'stack' - the main vertical pipe that carries waste - has to be at a certain radius, specified by British Standards.
This allows the flow of waste to follow a bend and then flow horizontally and into a drain, where the flow decreases - a phenomenon known as hydraulic jump. This is affected by the initial speed and amount of liquid flowing, as well as the location of pipes leading from toilets into the main stack.
But in addition, for buildings above 20 storeys, the bottom two toilets have to be on their entirely own stack - otherwise things can go badly wrong.
If this second system is not in place flushing a toilet at the top of the building will lead to something very unpleasant spewing - at speed and with force - out of the ground floor conveniences, the direct result of the "bounceback."
When designing a toilet system for a skyscraper, the first thing to consider is the number of appliances in the building - each of which is given something called a discharge unit.
A discharge unit is a measure of how much fluid any given appliance is expected to produce - for example a flushed toilet will create more waste than someone brushing their teeth in a sink.
The next thing to think about is the use the facilities are likely to receive - for example office toilets are used less than those in hotels.
A complex formula featuring these two numbers eventually gives you the litres per second of liquid you can expect to get rid of down the toilet system, which in turn tells you the size of waste pipe that will be needed.
For example, a ten storey domestic building with four toilets, two handbasins, two baths, two showers, two sinks and two washing machines on each floor will give out 5.87 litres per second - and need a 125mm diameter waste pipe to get rid of it all.
This compares with a normal house, which produces approximately 1.2 litres per second and needs a 110mm pipe (4ins).
The amount of flow is worked out by multiplying the number of appliances in a building by their given discharge unit and adding these figures together. We then find the square root of this number, and multiply this by a 'frequency factor'. For a house the frequency factor is 0.5, but for a hotel this is 0.7 and for public toilets 1.
Therefore, in a house with a bath shower and two toilets, the workings out would look like this:
2 Hand basins (0.3 x 2 = 0.6) + 1 shower (0.4) + 1 bath (1.3) + 1 kitchen sink (1.3) + 1 washing machine (1.2) + 1 toilet (1.5) = 6.3 discharge units.
To work out the flow of liquid through the pipes, at a rate of litres per second, we must find the square root of 6.3 which is 2.5. This is then multiplied by the frequency factor, 0.5 for domestic use, giving a flow rate of 1.25 litres per second .
Pipes are normally made from PVC - but in bigger buildings, particularly in London, at least the bottom section of the stack is cast iron to protect from rats, fire and the sheer pressure of waste flowing down them.
Woe betide any drainage expert who gets it wrong.
Mr Briggs said: "Hotels are a particular problem because everyone checks out at 10am - and everyone goes for a wee before they leave, so you have to deal with that.
"The new Wembley Stadium was also really hard, because shortly after at the final whistle, pretty much every toilet in the stadium would flush at the same time."
Here's how the toilets would work on some the world's most famous buildings, and how mechanisms differ depending on the country conveniences are in:
London's iconic 1,000 ft pyramid building combines restaurants, offices, shops and a hotel so is likely to have a pretty high discharge unit and frequency factor.
The system could be expected to deal with at least 60 litres per second - although this would likely be split between several stacks.
"In all my years I have never had to use a pipe wider than 160mm," said Mr Briggs.
Burj Kahlifa in Dubai
The world's tallest building is an enormous 163 storeys in height - 35 storeys higher than the next biggest tower.
And the pipes used to remove waste are a huge: around 600mm in diameter.
But they have to be because users of the building get through a whopping 250,00 gallons of water every day.
Empire State Building
New York's most famous building stands at an impressive 103 storeys.
While the principles of poo removal remain the same - US engineers use imperial measurements, which could leave UK experts scratching their heads.
"I'd expect the gradient of bend at the bottom of the stack to be roughly the same," said Mr Briggs, "although that is specified by British Standards".
"But Americans still use imperial measurements, so their pipe sizes would be in inches and feet."
Most tall buildings in the UK require a second pipe running alongside the main one to allow air to flow - and bad smells to escape from the top of the building.
However many buildings are now using a special valve which negates the need for the second pipe - allowing air in and the whiff out via a new kind of technology.
"It's possible the Shangai Tower would use an alternative valve arrangement," said Mr Briggs.
"Although it would depend on whether they had a US or UK consultant on the project."