In the heart of the unfinished building there's plain, cold concrete floors, waylaid builders' tools, scaffolding reaching for the very tall ceiling. There are leaks where the rain is coming through and wooden planks to step on to keep shoes dry.

As I take in the sight of the one useless red bucket, abandoned by builders and catching some of the fat raindrops, I can barely believe I'm in Barcelona, Spain. Isn't it always hot and dry in Spain? And I really can't believe I'm standing inside the Sagrada Familia, one of the world's most celebrated Roman Catholic cathedrals.

It's also one of the world's most visited - and chaotic - building sites. Designed by Barcelona's favourite architect, Antoni Gaudi, the Sagrada Familia was started in 1882 and workers have been going hammer and tongs ever since, decades after Gaudi himself was killed by a tram in his 70s.

I try to understand so many years dedicated to a cathedral and am amazed to think of where New Zealand's history stood at 1882 - we'd only just begun!

My boyfriend and I walked up to see the stunning Sagrada Familia on our first day in Barcelona and as we stood wide-mouthed, thinking what the..., we realised that one page of text in our travel guide (and standing wide-mouthed thinking what the...) would not be enough to settle our curiosity. A proper tour was called for.

The facade of the cathedral is indescribable and I've since found even pictures don't speak enough words on this one. It is just a gob-smacking creation and it's hard to believe there's still so much work left to do.

The next day we head up there again and meet our tour guide, who has to take about 15 people around in the rain and explain 126 years of construction history in 45 minutes. She does a good job and I'm madly scribbling in my little notebook.

As we look at the oldest facade, known as the nativity scene, it's amazing to think that Gaudi had these plans, models and ideas and that a city of people backed him up. The nativity facade looks like one of those glass bottles people cover with melted candle wax. It's so dense with droopy sculptures, it feels as if the whole facade is going to drip away eventually. Up close you can see that every droop is actually an incredible sculpture - telling a thousand tales.

In complete contrast to the aged sculptures on the nativity facade is the newer facade - a homage to Jesus' death. These were designed by Josep Maria Subirachs and they are the most moving sculptures I've ever seen. Each face is hallowed and haunted, with so much expression I could have stared at them for hours. It's hard to pick which is better out of the two facades but perhaps the third (still not built) will trump them both. And the interior is pretty special too.

Gaudi designed the inside of the cathedral to resemble a forest. Its column are thick and made to look like tree trunks - they rise upwards at an angle. And there's a beautiful ceiling with 'skylights'. At the moment this just means us tourists are getting wetter than we'd like but when the Sagrada Familia is finished, the skylights will mean worshippers can look up and see the stars twinkling - such a pleasant thought!

There's so much of the cathedral still under construction. I almost can't connect with the idea of a building having so many years of history and still having such an interesting future. Gaudi once famously quipped that 'my client is not in a hurry' but I'm sure the people of Barcelona would just love to see the finished result in their lifetime.

As the tour comes to an end, our guide shows us how only a small percentage of the stained glass windows have been completed. There are four enormous walls of plain windows still to be decorated.

She says experts expect the cathedral to be completed in 20 years and as our tour group raises skeptical eyebrows, I give her the benefit of the doubt. I'm already imagining returning in my late 40s to see those windows - and stars - for myself. Gaudi's 'client' was in no hurry, and I guess I'll have to wait a few years too.

Check out more photos from my trip in my Flickr album.

Find out about Barcelona.

Read more of Kelly's blogs.