Three bridges, three centuries, three stories.

The Queensferry Crossing is finally open with the first motorists having travelled over the water in vintage, modern and electric cars, tooting their horns on their way.

It is the third in a trilogy of crossings built over the cool grey waters of the Firth of Forth, with the first, the Forth Bridge which opened in 1890.

Described as one of the finest pieces of engineering in existence, the Victorian steel structure, 200 trains use the bridge every day, carrying three million passengers each year.

The second Forth Road Bridge followed in 1964. At this time, it was the first bridge of its kind in the UK, the longest outside the US and the fourth longest in the world.

The Queensferry Crossing has been collecting some new wold records of its own.

The 1.7-mile structure is the longest three-tower, cable-stayed bridge in the world and is the largest to feature cables which cross mid-span.

This design provides extra strength and stiffness, allowing the towers and the deck to be more slender and elegant.

It landed a new world record in 2013 when construction workers achieved the largest continuous underwater concrete pour.

The 24-hour non-stop operation successfully poured 16,869 cubic metres of concrete into the water-filled south tower caisson over 15 days.

Although the central tower of the bridge is constructed on top of Beamer Rock in the centre of the Firth of Forth, the two flanking towers are each actually founded below river bed level on a giant steel cylinder, the largest being the height of the Statue of Liberty.

The structure rises 683ft above high tide, equivalent to approximately 48 London buses stacked on top of each other.

Described by its constructors as one of the world's great bridges, it is the largest of its type.

One of the project managers described the operation as "civil engineering at its most raw and exciting" because of the scale of construction and the risks involved.

The bridge used 150,000 tonnes of concrete, nearly as much as the London Olympics, and 35,000 tonnes of steel - the equivalent of 200 jumbo jets.

The combined steel required for both the north and south viaducts weighs 7000 tonnes, enough to make 23 Kelpies - the huge horse-like landmark found in Grangemouth.

It is estimated construction of the bridge involved approximately 10 million working hours.

Its goal was to serve as a suitable replacement to the ageing Forth Road Bridge, which unlike its Victorian forebear was showing signs of deterioration.

In 2004, an internal inspection of the main cables found that corrosion had resulted in a loss of strength, giving rise to fears that significant traffic restrictions might be required in future to allow for repairs.

At that time some 70,000 vehicles used the bridge every day and it was one of the most vital economic arteries in Scotland.

The Queensferry Crossing was the solution, after a period of testing all sorts of other options, from underwater tunnels to hovercrafts.

For most of its early days, though, work on the bridge happened very much beneath the waves.

Before the bridge towers could rise from the Forth, a concrete plug had to be out in place on top of the estuary bed 14 meters below water-level.

The Centre Tower had a head-start. Its foundations were only around 6ft below the waterline, starting from the rocky outcrop known as Beamer Rock.

The North and South Tower foundations required the use of huge steel caissions.

Derived from the French for "casing", essentially a caisson acts as a mould, enabling the concrete foundations to be formed under water and for sub-marine work to be carried out "in the dry".

The largest is about 100ft high by 100ft in diameter - approximately the size of an eight-storey building.

It weighs a massive 1200 tonnes, making it one of the largest steel caissons ever sunk down to the seabed anywhere in the world.

Once you start sinking 1200 tonnes of steel, you only have one shot at getting it in the right position, though.

What follows is a fairly tense moment, when the margin for error is an extremely tight 2.5cm.

They were sunk vertically, any tilt being corrected by careful ballasting and excavation, until they were in place.

From there the towers could begin to rise.

The new crossing will now take most of the vehicles that currently travel over the 53-year-old Forth Road Bridge.

It has a design life of 120 years but could last longer as it has been "designed for maintenance" to ensure it runs smoothly for decades.

The team who built it say it has been a "privilege" to work on it and their own efforts, in turn, were recognised at the opening.

Three days before it opened to the public, First Minister Nicola Sturgeon switched on the lights and a projection was beamed over the entire bridge to mark its completion.

Addressing workers on the crossing, she thanked them and told them they had "made history".