All over the world, there are iconic pieces of historical architecture that were in such a bad state of disrepair they could have literally fallen apart piece by piece. Some of these buildings are still at risk of total ruin. Here we take a look at some of the most well-known structures and examine what, if anything, has been done to restore them to their former glory. As you'll see, some of the costs involved are astronomical.

Built in the early eighteenth century, the Spanish Steps, which link the Piazza di Spagna to the Piazza Trinità dei Monti, is one of Rome's most popular tourist attractions. The millions of sightseers who have traipsed up and down the steps over the years, however, have taken the toll. The stairs were closed for ten months in 2016 for a €1.5 million ($1.7m/£1.3m) clean and restoration. The bill for this was picked up by Italian jeweller Bulgari. The steps are now almost like new – until the next few millions of visitors have made them crumble again.

Measuring more than 13,170 miles, over a third of the Great Wall of China has disappeared since construction began on it in the third century BC. This is down to natural erosion, tourists taking pieces of it home as souvenirs and locals stealing bricks to build houses. A crowdfunding campaign was set up in 2016 to raise $1.6 million (£1.1m) to help restore it. The work has begun, but some of the modern methods employed have sparked controversy.

The Eiffel Tower was built as a temporary structure to mark the 1889 Universal Exhibition in Paris. It might be still standing over a century on, but it's starting to show its age. The first floor was closed to the public for two years in 2014 and now a further €300 million ($330m/£270m) restoration project is underway. Some of this is going towards overhauling the original lift workings and modernising the lights, as well as improving security and the visitor experience.

Constructed more than 500 years ago, the brick dome of Santa Maria del Fiore in Florence remains the largest ever built. However, the dome is cracking under its own weight and no-one knows exactly how it was constructed, as there are no sketches of the structure. Even though the cracks are meticulously monitored and the dome isn't expected to fall down any time soon, a solution to the restoration problem isn't in sight, let alone how much it would cost. Scientists are currently using cosmic rays to collect data about the exact architecture.

The Taj Mahal is almost 400 years old and, unsurprisingly, it has started to look its age. All is not lost though; all it actually needs is a mud pack. Packs of non-abrasive and non-corrosive mud have been started to be applied to its marble to restore its whiteness and gleam. The cost of the eco-cleaning process, which is carried out bit by bit, has not been made public but is thought to be significant.

Queen Elizabeth II's official London residence Buckingham Palace is currently in the middle of a 10-year renovation programme to restore its crumbling structure and replace its boilers, much of its wiring and pipework. Dating back to the 17th century, the property hasn't been decorated since the 1950s. The works are estimated to be costing $495 million (£370m). Restoration of the Queen's bedroom is planned from 2025.

The World Heritage site of Angkor Wat is believed to be the largest religious building on the planet. Millions of dollars are thought to have been spent on cleaning and repairing the ancient Cambodian temple over the last 30 years, with India picking up most of this cost. The renovations were so extensive that parts of the building had to be dismantled and rebuilt.

An unusually cold winter caused parts of the 18th-century Trevi fountain's ornamental cornice to come away. Once work began to fix this in 2014 further necessary repairs were revealed, which led to the bill rising. Italian fashion house Fendi stumped up the €2.2 million ($2.6m/£1.9m) for the repairs to the crumbling structure.

Scaffolding covered the Colosseum for much of the last few years as it underwent the first refurb in its history. It was built nearly 2,000 years ago, so a makeover was well overdue. The work to return the building to its original colour by carefully removing dirt and grime took 33 months and once again an Italian fashion brand stepped in and picked up the bill, with Tod’s covering the $27.6 million (£20m) costs.

The restoration of the Palace of Versailles is a never-ending job. Since 1978, the French government has spent millions of dollars restoring the hundreds of rooms in the main building and those rooms in the Petit and Grand Trianons. The living quarters of the palace's royal tenants have also been overhauled, as have the gardens. After the restoration of the Queen's library the Royal Chapel is now one the latest areas to have the builders in and due to be finished in 2020.

Built by the Romans, parts of Hadrian's Wall's central section was put on English Heritage's 'at risk register' not so long ago, but stonemasons entered the World Heritage site in 2013 to restore its crumbling core. They began taking apart and then rebuilding the wall at a cost of $669,000 (£500k).

The city of Petra was carved into the sandstone cliffs in the second century AD. It was abandoned and forgotten about for centuries and discovered again in the 19th century. Restoration works have been ongoing in recent years that have benefited from international support. In 2015, the US ambassador to Jordan awarded a $200,000 (£149k) grant to help with the second phase of conservation at the Temple of the Winged Lions.

It is estimated that repairs to the UK's Houses of Parliament will cost between $4.3 billion (£3.5bn) and $6.9 billion (£5.6bn). The essential restoration works are required to reduce the risk of fire, fix damp and leaks and replace cabling. The parliament will even have to move out to temporary premises while the work is carried out, which is estimated to start in 2025 at the earliest. The bill is so vast that parliamentarians have questioned whether a cheaper solution could be found. Repairs to the iconic clock tower that houses the famous Big Ben bell have already begun (pictured).

The Hanging Gardens of Babylon archeological site is one of the seven ancient wonders of the world. It suffered from neglect during the rule of Saddam Hussein, but since 2008 the World Monuments Fund has been working with Iraq’s State Board of Antiquities and Heritage to conserve the fragile archaeological remains. The US State Department and various foreign charitable foundations have contributed hundreds of thousands of dollars to its preservation, but more is still needed.

The Acropolis of Athens has had the restorers in for decades. Works began on rescuing it from decay in 1975 and are not expected to be completed until 2025. In 2005 it was announced that a further €70 million ($81m/£61m) was required to finish the job. Despite Greece's financial crisis, work has never ceased on the site and it continues to receive funding from the national government and the European Union.

UNESCO awarded the Royal Palace of Caserta World Heritage status in 1997. The world's largest palace by volume was in such a bad state of disrepair from water damage and the growth of vegetation that parts of the main facade actually collapsed in 2012. This triggered the start of a €15 million ($17m/£13m) restoration programme funded by the European Union.

Djingareyber Mosque was built in the 15th century from the earth that surrounds it. Until recently the only repairs consisted of fixing the mud rendering. This was funded following an appeal from the Iman to the local population who contributed funding, materials and labour. Islamist extremists damaged part of the mosque in 2012. UNESCO, the World Bank, the European Union, Switzerland and the USA have funded an $11 million (£8m) restoration project.

Unstable soil, pollution and poor water drainage had left the ruins of Pompeii facing further destruction, but the European Union came to the rescue in 2012 with a €75 million ($88m/£66m) grant. The Italian government then provided a further €30 million ($34m/£26m) for what has become known as The Pompeii Project. Over 200 technicians are now on site preserving and restoring the ruins and uncovering more archaeological treasures for future generations to enjoy.

Built in the late 19th century, the House of Wonders was one of East Africa's most modern buildings at the time and the first to have electricity and a lift. It was made a UNESCO World Heritage site in 2000 and most recently operated as the Museum of Zanzibar focusing on Swahili culture. However, in 2012 part of the eastern corner of the building collapsed and the museum moved to a different location. The house is now closed to visitors due to disrepair, still awaiting a costly repair operation.

Built by the Spanish in the 16th century and heavily shelled during World War II, it was feared that the ancient building of Fort Santiago in Manila could become a shopping mall as commercial advancement was taking over. Some of the old moats surrounding the building were filled and turned into a golf course. Now there is more local interest in preserving the Fort as a historical artefact and restoration works have been carried out in recent years.

The Alhambra Palace has just undergone emergency repairs to ensure that the historic building remained safe for visitors and to preserve its ancient artefacts for future generations. The restorations cost hundreds of thousands of euros: €350,000 ($414k/£310k) alone was spent on returning the Court of the Lions (pictured) to its original splendour. A proportion of the restoration fund was also spent on determining the causes of the original damage to ensure that it doesn't happen again.

King Ludwig II's Neuschwanstein Castle is what fairytales are made of, but around 1.5 million people visit it every year and all that footfall has taken its toll. Sunlight has also damaged the furnishings and fabrics and the harsh mountain weather has eroded some of its limestone facade. A €20 million ($22m/£18m) restoration project is now underway to restore it to its former glory; this is due to be completed in 2022.

Dating back to the 17th century, the San Miguel Mission is said to be the oldest church building in the US. It urgently needs structural repairs to prevent further damage to the building and is fundraising for money to help improve the visitor experience and open up more of the building to visitors. Its fundraising target to cover all of this is $14.8 million (£11m).

Thought to be the oldest building in South Africa, the Castle of Good Hope is a 17th-century fortress in Cape Town. It was declared a national monument in 1969 and went on to house the country's Military Museum. It has recently undergone a two-year $6.6 million (£4.9m) restoration programme to deal with damp problems and leaks. Murals were also refurbished and specialist plastering was repaired.

Its lean is what made the Tower of Pisa famous. It has appeared to defy gravity, leaning at a 10-degree angle ever since it was constructed in the 13th century. Cement was injected into its foundations in the 1920s in a bid to stabilise the tower and it was shut for a decade in the 1990s while further supports were added. This latest $33 million (£25m) project actually helped to correct the original lean and it has been discovered that it has straightened further since.