There was only one thing to be done. The waterfall had to move off centre.

"Every three minutes, the train coming through would have gotten a train wash," Safdie quips.

Not only that, the train track ran straight through the centre of the dome - right through where the waterfall would have been.

Changi Airport's Skytrain track was already there - and the entire Jewel complex had to be built around it.

From the beginning, there was one key obstacle to architect Moshe Safdie's vision of building a domed glass complex with a waterfall cascading through the centre of its roof.

You go out, you look up, you see it's not in the middle. And that was a geometric nightmare.

"But I think it also made it so much more beautiful and created a tension in the geometry."

"Doing an asymmetrical toroid was very complex - moving the toroid off centre because the train is in the middle," Safdie says.

In the middle is an 11-metre-wide hole - the oculus - except that it isn't actually in the middle.

"We calculated that the surface would bring, on a good Singapore rainfall – and you got a lot of them – 12,000 gallons (about 45,430 litres) a minute. Well, that's something to reckon with. And water can be recirculated and used."

Intrinsic to the idea of the toroid was the idea of a waterfall running through it: Collecting the rainwater that falls on the entire building and funnelling it into the structure through the oculus.

"I think if this was a dome you would look up and say, yeah I've seen a dome before. They'd probably take fewer Instagram pictures."

"There's something about a toroid," Safdie says. "There's something about the fact that it turns on itself, the fact that it's hanging there and that the water's coming through it, that makes it very magical compared to a dome."

And finally, the air-conditioned terraces were lined with thousands of trees, plants, ferns and shrubs brought in from around the world.

The world's tallest indoor waterfall was engineered, with tens of thousands of litres of water cascading over seven storeys.

A massive canopy rose over it, formed with more than 9,000 slabs of glass, no two of them alike.

Workers dug five storeys deep - the deepest basements in the whole of Changi Airport.

Over the next five years, Jewel took shape, right in the heart of Changi Airport aerodrome, without disrupting vital operations, services, traffic or infrastructure around it.

"And here you're discovering it inside a building."

He notes that nature works in much the same way: The landscape collects the rain; brings it into valleys, brings it into rivers, brings it into waterfalls.

"Now, you're creating a garden which you want to be like a magical place of nature," Safdie says.

"If the water had hit the main valley floor, we would all be wet now," Safdie says.

and to the bottom of basement 2.

As much as 37,850 litres of water flows through the oculus every minute,

This is where the waterfall begins.

"It's a sensational experience," Safdie says. "All that power of water is right in front of your nose but you can't hear it, because it's absolutely sound-proof."

Inspired by aquariums, the column is made of more than 40 sheets of acrylic, fused to form a seamless screen.

It became a major feature of the waterfall: an acrylic funnel spanning basements 1 and 2, creating a buffer for the water as it hit the bottom.

"We realised we've got a hell of a lot of water coming down with a bang, so to speak; we realised we had to buffer it somehow, so that led us to say, 'Let's enclose it.'"

At level 5, another series of pump rooms sends the water up the surface of the glass dome through a network of pipes on the facade.

The water goes into massive tanks in the basement, where it is filtered before being pumped back up to level 5.

Take a close look at the "fins" on the facade - there are 24 pipes concealed within them.

The aim is to make it look seamless.

WET senior designer Tim Hunter says the water design firm took inspiration from the curving roof structure of Jewel itself.

From there, the water falls freely to the ground.

The pipes flow down to a chamber known as a manifold, which runs around the entire oculus, Ashith Alva, Jewel's head of project design explains.

"As the roof bends inward, the glass canopy seamlessly transitions into water as it continues its journey down into the heart of Jewel."

The water falls as a continuous parabolic arch rather than a straight line, he points out.

The biggest challenge was to create a place that's good for the trees and good for people.

Alva notes that there had been concern about the heat potentially generated as a result of the glass house effect.

"If you wanted to design a place where nature is indoors, you've got to give it sun. If you give it sun, it's a glass house. That's what Jewel is," Safdie says candidly.

As such, each panel in the facade is made of three layers of glass - an outer layer, a 16mm air gap and a double-laminated layer. This reduces the amount of radiant heat that gets in the dome, Alva says.

"So those sitting there get their tush cooled nicely."

This is why the benches all over the Canopy Park have an extra purpose. The air-conditioning system is concealed within them, he reveals.

"You don't want to bring the cold air from above. You want to bring it from below," Safdie adds.

Only the first 2 metres or so of each level is cooled - "about human level", Alva explains.

To be more energy-efficient, the dome isn't cooled in its entirety.

Because chilled slabs absorb the heat, less air-conditioning is required.

The chilled slabs are used all over the areas likely to get the most direct sunlight - the top level, as well as on the ground floor around the waterfall.

They are literally chilled floors - pipes with chilled water going into the flooring system, to absorb the heat.

If you touch the floors in the Canopy Park on level 5, you'll notice that they are cold.

Here, people were number one. 24 degrees, comfortable humidity. What kinds of plants were going to thrive in these conditions, was the question.

"But we used them to make the benches for the building, so these benches were the trees that were in the parking lot on the site."

The car park that Jewel was built on had been shaded by large, mature trees. "We were going to use them in the building, but it turned out we couldn't because they didn't survive," Safdie says.

However, one part did not go according to plan.

In all, there are 2,000 trees and palms and more than 100,000 shrubs from 120 species.

"I want things that have delicate foliage, that resonate in the light … and so we aimed for plants, many of which are not familiar in an interior environment, but remind you of being out there in a mountain forest. I like being in a mountain forest."

"I didn't want the plants that are sort of associated with interior planting - tropical plants that had been used over and over and over, and mostly selected for their indestructibility," Safdie says.

Such plants generally come from tropical climates and high altitudes, so the team established the geographic range where they would be sourced from: Malaysia, Thailand and Australia, as well as China, the United States and Spain.

Jewel Changi Airport: FAQ

HOW TALL IS JEWEL CHANGI AIRPORT?

The glass dome of Jewel is 37.08m high. It would have been taller, had it not been for the height limitation. "We had to stay below the radar," Safdie quips. "There’s a very specific line of the (control tower) radar we could not exceed." He reflects that the height limitation was probably the toughest one for the team. "We could have used more height to get more curvature into the dome," he says. "The dome is quite flat because of that."

HOW BIG IS JEWEL’S ROOF?

Jewel’s roof facade is 200 metres long and 150 metres wide.

HOW HEAVY IS IT?

The roof weighs 3,500 tonnes, equivalent to six Airbus A380 planes.

HOW TALL IS THE WATERFALL?

The waterfall is 40m tall - taller than the building itself, because the waterfall extends into basement 2.

HOW MUCH WATER FLOWS THROUGH THE WATERFALL?

During the day, the water flows at about 37,850 litres per minute, which is known as the "high flow", Alva says. In the evening, during the light and sound show, the flow is reduced to about 5,680-7,570 litres per minute so that the light projections can be seen more clearly.

HOW IS THE WATERFALL CONTROLLED?

The nerve centre for the Rain Vortex is locked behind a nondescript door in the basement 3 car park. "This is the heart of the operations of the water feature," Alva says. "This is where we have our filters, our control panels, all the various mechanisms that control the water feature."

HOW BIG ARE THE RAIN VORTEX WATER TANKS?

The main tank at basement 3 has a capacity of 420 cubic metres. Beneath it, in basement 4, lies a much larger tank of 1,670 cubic metres - four times the size of the main tank. While the amount of water at basement 3 is kept constant, there is an overflow mechanism which allows excess rainwater to flow into the larger tank beneath.

HOW CLEAN IS THE WATER?

Basement 3 is also where the water is filtered. A brominator is used to add bromine to the water. "As with all water tanks and systems, it’s about algae formation that we’re trying to avoid," says Jeremy Yeo, head of user experience at Jewel Changi. "We want to make sure that the water that is getting pumped up is very clean and pure," Alva says.

WHAT HAPPENS WHEN IT RAINS?

The building has rain sensors, says Alva. When triggered, the water feature will shut off and when the rain is heavy enough, it flows into the basement 3 tank.

WHAT HAPPENS WHEN THERE IS TOO MUCH WATER?

When the tanks are full, excess rainwater is used for irrigation or diverted for other uses.

WHAT HAPPENS WHEN THERE ISN’T ENOUGH WATER?

If the required water level is reduced due to evaporation, it is topped up using NEWater.