There is a trapdoor on Christ's right shoulder.

To mount the steps and slowly, fearfully peer out is to see the world through the eyes of a bird, or even a god. Far below, white blocks of flats and offices cluster among folds of tropical green.

Down there are the poor in the favelas, the rich in the luxury high-rise apartments, the homeless, the famous football stadiums and Guanabara bay with its scattered islands and boats. Beyond the sands of Copacabana and Ipanema, the limitless Atlantic ocean.

To the left, standing twice a man's height, is the slightly bowed head of Christ, also looking down on the beauty of the city.

But unlike the forests or the ocean, this statue was the work of man and will not last for ever. Close up, the toll of 83 years of weathering is starkly apparent.

Unnoticed by the 5,000 who visit the landmark every day - and see it only from a distance - the surface is a patchwork of worn mosaic tiles resembling the skin of an aged reptile.

Lightning storms have been chipping away at it. In January, two direct hits in eight days blasted off a middle fingertip and scorched the back of the head, sparking a race to patch up Rio's favourite picture-postcard scene before the World Cup in June.

Such is the statue's popularity that even at 8am there is a babble of tourists taking photographs and enjoying the view. For them, the only sign of anything wrong is the scaffolding that leads up the 8m (26ft) pedestal to a discreet entrance in the hem of the cloak.

If the statue had a right ankle, the door reserved for workers would open into it. Once inside, the hubbub disappears.

There is little natural light and only a few bare bulbs. Flights of open stairs make their way up through the centre, between the criss-crossed concrete supports that give the statue its strength.

A thick skin of reinforced concrete means the inside is cool, despite the summer heat outside.

Climbing up, a number is crudely painted on the wall of each of the 12 floors in what feels like an abandoned, dusty warehouse.

There is no sign that this is the inside of Christ the Redeemer until an upper level, where a roughly shaped heart bulges from the inside of the chest. It is covered in the same stone mosaic as the outside of the statue - where the outline of the heart can also be seen - the only delicate detail in an interior that's otherwise rough around the edges.

From the top of the last set of stairs, a vertical steel ladder leads to a tunnel in the statue's arm. A dark, narrow passageway then stretches all the way to the fingers.

The only way to inspect the damage caused by the recent lightning strikes is to go out - through the top of the 30m-high statue.

The workers who crawl out of the holes in the arms, shoulders or head use ropes to tether them as they abseil down the torso or inch along the 28m-span of the arms, the city spreading out before them, far below.

Brazil's tropical location makes it one of the lightning capitals of the world, which was never going to be good for a statue on a sharp 710m-high granite peak.

There are two, maybe four, direct hits each year, according to the Brazilian Institute of Space Research. Most cause no damage.

But recent storms have been unusually violent. "In the past few years, there have been some cases of storms registering more than 1,000 lightning bolts, which did not occur previously," says Dr Osmar Pinto, chief of the institute's atmospheric electricity group.

"It is necessary to review the structure of the statue periodically and revise the earthing system of the lightning rods."

The statue has a conductor that covers its head in something resembling a crown of thorns, and stretches down each arm to the hands.

Part of the work now under way is to extend the lightning rods to the statue's fingertips. Improving the earthing of the rods is just as important.

Effectively earthed, there would be less risk of damage to the statue in the immediate vicinity of the conductor. But earthing is tricky at the top of a big granite rock, as granite itself conducts electricity poorly.

The damage caused by January's dramatic lightning strikes was more serious than usual says Paolo Dal Pino, president of tyre company Pirelli in South America, which is footing the 1.9m real (£500,000, $790,000) bill for the current round of repairs.

"For a monument like this, that hosts two million guests a year - it's going to be three million people probably in 2014 - a place like this, damaged, is something that cannot exist," he says.

With bigger storms, and the statue getting older, it's possible that such repair jobs will be needed more frequently. But the original pale grey-green stones that make up the mosaic surface have run out - so there is a possibility that the statue will gradually turn a darker shade.

Already, years of piecemeal repairs mean the statue is a patchwork of varying shades of grey, blue and green, when seen from close quarters. Future repairs will be further from the original colour, unless a new deposit of the stone is found.

Marcia Braga, the architect who led a restoration of the statue in 2010, says she faced difficulties finding the right stone. In the process of replacing 60,000 tiles, she rejected 80% of those supplied by the quarry.

"The idea is to do something as close to the original as possible because when you use different colours it's not a pleasant aesthetic," she says.

Reports that all the statue's six million stones will be replaced, and that Christ the Redeemer will change colour in one fell swoop, have been denied.

The next major renovation is expected in 2020, 10 years after the last. As yet no decision has been made on how many tiles to renew, but any new stones used will be a deeper shade, "a different, a darker green", says spokesman for Brazil's National Institute of Historic and Artistic Heritage.

"The stones of Christ are hard to find."

The stone tiles that cover Christ the Redeemer were one of the last pieces of the design to be finalised.

According to the project's Brazilian architect, Heitor da Silva Costa - seated, in the picture above - it was the first time mosaic would be used on a statue.

The original idea for a monument to Christ came from a group of Brazilians who, in the wake of World War One, feared an advancing tide of godlessness. Church and state had been separated when Brazil became a republic at the end of the previous century, and they saw the statue as a way of reclaiming Rio – then Brazil's capital city – for Christianity.

The first proposal was for a bronze statue of Christ on Sugar Loaf - the giant lump of rock with a smooth, curved summit that rises out of the ocean at the entrance to Guanabara Bay. But it was soon decided that Corcovado ("hunch back") - a peak in the forested hills behind the city - was a better location.

Da Silva Costa, whose design was chosen in February 1922, imagined the statue facing the rising sun: "The statue of the divine saviour shall be the first image to emerge from the obscurity in which the earth is plunged and to receive the salute of the star of the day which, after surrounding it with its radiant luminosity, shall build at sunset around its head a halo fit for the Man-God," he wrote.

His initial design showed Christ carrying a large cross, which he hugged to his body with one hand, while holding a celestial globe with the other. Some people made fun of it, calling it "Christ with a ball".

As he studied the Corcovado from various vantage points in the city - at that point topped with a radio transmission tower erected by Westinghouse - a new design took shape. In this new version, developed with artist Carlos Oswald, Christ was himself the cross, his outstretched arms signifying the redemption of mankind at the crucifixion.

But the new design introduced new challenges. Da Silva Costa had already concluded the structure would need to be huge, to be visible from the city centre 4km (2.5 miles) away. It would also have to be immensely strong, to support the massive arms. Da Silva Costa decided on reinforced concrete, "the material of the future" as he saw it, and headed for Europe in 1924 to seek help from the leading French engineer in the field, Albert Caquot.

While there, he also met a number of European sculptors. Antoine Bourdelle, who had worked with Rodin, was one of those approached to make a 4m-high scale model based on Oswald's drawings, but it was French-Polish sculptor Paul Landowski who received the commission.

Oswald's sketches were already voguishly art deco – but Landowski intensified this stylisation, working particularly on the head and hands, which he produced full-sized in clay, to be shipped to Rio where they were reproduced in concrete.

By 1927 a preliminary steel frame had already been erected on the top of Corcovado and yet the problem of the statue's finish had still not been solved. Da Silva Costa regarded concrete itself as unacceptably rough and crude.

"We were marching towards the inevitable artistic failure, without being able to go back," he wrote later.

Inspiration came in an arcade which had recently opened on the Champs Elysees, where, after work one evening, he saw a fountain covered in a silvery mosaic.

"By seeing how the small tiles covered all the curved profiles of the fountain, I was soon taken by the idea of using them on the image which I always had in my thoughts," wrote Da Silva Costa. "Moving from the concept to the making of it took less than 24 hours. The next morning I went to a ceramic studio where I made the first samples."

For the material, Da Silva Costa chose soapstone, according to his great-great-granddaughter, Bel Noronha, partly because it had been used by the 18th Century sculptor Aleijadinho ("the cripple") in the state of Minas Gerais, just north of Rio. After losing his fingers to disease, Aleijadinho miraculously continued to carve ornate statues using a hammer and chisel tied to what was left of his hands. That these were still in good condition 120 years later, in Da Silva Costa's view, testified to the stone's durability.

He selected a pale-coloured example from quarries near the city of Ouro Preto, where Aleijadinho had worked – unaware that eight decades later it would run out.

Small triangles of the stone, 3cm x 3cm x 4cm, and 5mm thick were then glued on to squares of linen cloth by women volunteers in one of the parishes at the foot of the Corcovado.

They often added a personal touch to their work by writing messages or their boyfriends' names on the back of the tiles.

"I wrote many wishes on the soapstones," said one of the workers, Lygia Maria Avila da Veiga, in a film shot by Bel Noronha.

"They are up there, up there on top."

There is still a strong connection between the statue and those who live closest to it.

During Carnival, a street party called Christ's Armpit, or "Suvaco do Cristo", weaves its way beneath Corcovado in tribute to the outstretched arms overhead. Crowds of dancers and drummers samba through the streets wearing T-shirts bearing the image of Christ.

For those who live in the Santa Marta favela nearby, the monument is constantly visible.

"I'm aware of it every day – it's a nice thing to have on the landscape," says Silvana Castro da Silva. "It's beautiful at night when it's all lit up."

A mile away, as the crow flies, the figure of Christ appears like a white cross gleaming on the hill top. Or on special occasions it may be illuminated in a coloured light – lilac for Mother's Day, pink for a breast cancer charity or blue for an autism awareness day.

"We're privileged to have a view of Christ – it's a symbol of our community," says Daniel Nascimento who sells a popular Brazilian sorbet made from the Amazonian acai berry.

"I'm not religious so for me, it's just one of the most beautiful places in Rio. There's a trail here up the hill to Corcovado that only the residents know about – it's something special."

Despite the religious inspiration behind the statue, it was never seen exclusively in a religious light. Count Celso, one of the instigators of the project in the 1920s, described the completed work as a "monument to science, art and religion".

"It's a religious symbol, a cultural symbol and a symbol of Brazil," says Padre Omar, rector of the chapel in the base of the statue. "Christ the Redeemer brings a marvellous vista of welcoming open arms to all those who pass through the city of Rio de Janeiro."

The image of the statue is reproduced everywhere - in graffiti art, sand sculptures on Copacabana beach - and even on skin.

Edilson Porfirio Dantas, who has lived in Rio for 18 years, has a Christ tattoo covering his entire back. "It took eight hours to complete," he says. "I'm not from Rio but the city is in my heart and Christ is beautiful."

According to Marcio Roiter, president of Brazil's Art Deco Institute, Christ the Redeemer has multiple meanings - "for each person a different meaning".

As he sees it, it's not explicitly religious. "It's more like somebody giving you a hug - welcoming you."

His words echo a 1969 song, That Hug, by one of Brazil's most famous singers, Gilberto Gil, which is sometimes said to have been inspired by the statue's arms-wide pose.

After a spell as a political prisoner, the newly released Gil rejoices in the vibrancy of Rio, the girls of the favelas, the famous Portela samba school, the Flamengo football team, the Banda de Ipanema street parade…

"Hello, Rio de Janeiro - that hug!

"The Brazilian people - that hug!"

Like the statue, Gil embraces everyone - and is embraced in return.