So THAT’S why all the photographs are of the outside! Revealing the boring interiors of the Leaning Tower of Pisa after 300-stair hike to the top

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The Leaning Tower of Pisa took 199 years to build. And it appears by the time builders reached the rafters they had run out of steam.

Photos showing the tower's interior reveal it as a hollow shell.

It comes as a surprise to many, given the landmark's grand exterior, which is adorned with marble arches and intricate stone carvings.

Empty shell: The Leaning Tower of Pisa took 199 years to build - it appears by the time builders reached the rafters they had run out of steam, as this image shows

After climbing 300 stairs and paying a €15 entrance fee, visitors are greeted with bare bricks when they reach the bell tower. 

In 1987 the Tower of Pisa was declared a World Heritage Site by the United Nation's cultural organisation Unesco, but as fears grew that it would topple over it was closed to the public in 1990 and engineers worked to stabilise it for the next 11 years. 

Preservation: In 2010 restorers helped stabilise the Leaning Tower of Pisa's tilt by removing soil from beneath one side of its foundations

'The tower was on the verge of collapse, but we managed to stop the tilt and secure it,' said Giuseppe Bentivoglio, from the Opera Primaziale organisation that preserves the tower.

The tower was reopened to the public in 2001 and remained open throughout a restoration costing almost £6million - partly to keep tourists happy, but partly because the revenue from ticket sales helped pay for the upkeep. The tower attracts over one million visitors a year.

In 2010 restorers made the tower even more stable by removing soil from beneath one side of its foundations. Its angle was previously 5.5 degrees from the perpendicular, but is now only 3.99 degrees off straight. 

Experts say the seven-storey bell tower should now be safe from further intervention for at least the next 200 years. 

But how did the tower achieve its lean? The most respected theory suggests the tower began to sink after construction - which began in 1173 - had progressed to the third floor after five years.

The cause was a flawed design - it had a foundation that was only three-metres deep set in weak, unstable subsoil.

Landmark: In 1987 the Tower of Pisa was declared a World Heritage Site by the United Nation's cultural organisation Unesco, but as fears grew that it would topple over it was closed to the public in 1990

Back then that area of Italy was very belligerent, with various local land-grabbing factions jostling for position.

Because of the battles between Pisa and nearby Genoa, Lucca and Florence, the construction of the tower was put on hold for almost a century.

Thankfully this allowed enough time for the soil to settle - had there not been that length of break, many believe the tower would have toppled over centuries ago.

When tools were picked up once more, under architect Giovanni di Simone (who had built the Camposanto Monumentale, the fourth and last building to be erected in Cathedral Square) in 1272, the engineers built upper floors with one side taller than the other, in an effort to compensate for the tilt.

Because of this, the tower is actually curved. Construction was halted again in 1284, when the Pisans were defeated by the Genoans in the Battle of Meloria, and the seventh floor was not completed until 1319. Its stewardship at that point had passed to Tommaso di Andrea Pisano. 

Just seven miles from the Mediterranean Sea on the west coast of Italy, the tower, which weighs some 14,500 metric tonnes, is frequently battered by storms that have eroded and discoloured it.

The distinctive, yellowish stone came from the quarries of San Giuliano, visible from the top of the tower on the green hills behind Pisa.