Every year in early spring, Helsinki is the setting for one of nature's most fascinating phenomena.

The exact date it begins is impossible to predict, although the local inhabitants seem able to sense its arrival.

To outsiders it is as bizarre and frightening as any anthropological event on earth. The mass hysteria is perhaps comparable only with the Running of the Bulls in Pamplona or the recent G20 riots in London.

I was warned to keep my distance.

"When the city turns yellow" locals said, "go home and lock the door."

Needless to say, I ignored their advice. I wanted to bear witness to the event, and if possible capture it on film.

I wanted to shed light on this extreme human behaviour so that one day we might be able to better understand it, and in doing so better understand ourselves.

The phenomenon is called "Crazy Days" or Hullut Päivät in Finnish.

It is the biannual sale of Helsinki's favourite department store, Stockmann.

For local residents Stockmann is more than just a store, it is an institution. Founded over a hundred years ago, it is located in a grand old building in the centre of town and sells everything from good quality food to cars. Although its prices are higher than other shops, it is always busy.

Stockmann has no equivalent in New Zealand - Kirk's on Lambton Quay is similar, if on a smaller and less popular sale.

The interest which the sale generates is quite unlike anything I have seen before.

People are excited about it for weeks beforehand, speculating on chat forums about the date it will begin and what might be discounted.

Once the sale is on, the centre of the city turns yellow with the enormous banners hanging from the store and every second person carrying the distinctive bright yellow bags Stockmann uses during this period.

For someone unfamiliar with shopping in Helsinki, the frenzy it causes would make little sense. Even allowing for the good quality and service found at Stockmann, the prices of goods inside would not justify it.

But to locals it is still something worth getting excited about. The reason is simple: Helsinki is painfully expensive.

According to the Economist, in 2008 Helsinki was the fourth most expensive city in the world.

Every consumer item from petrol to bananas - with the notable exception of mobile phone rates - is guaranteed to make you wince at the checkout.

For example, if you want to go out for dinner in the centre - whether for Indian or Finnish food - it will usually cost at least 15€ (NZ$36) for the main course alone.

Given that the cheapest bottles of wine normally start at about 25€ (NZ$60), an expensive night out is inevitable even before heading for a bar for 5€ (NZ$12) pints of beer.

Rent is expensive (we pay 900€ a month for a 50sq metre apartment), as is public transport.

Clothes are dear as well and unlike in England and New Zealand there is no Argos or Warehouse to pick up cheap Chinese-made essentials for the back yard and beach (Stephen Tindall take note).

Perhaps most annoying of all, however, are the supermarkets.

Finland is dominated by two chains, both of which are very expensive. Just the other day I paid 56 cents (NZ$1.33) for a lousy banana.

To make matters worse, prices are still rising.

According to the newspaper HBL, the cost of a typical basket of food in Helsinki rose by 11 per cent in the last year alone. Just a few hundred kilometres away in Berlin, that basket costs half what it does here.

There may be a number of reasons why the cost of living is so high in Helsinki.

It seems that food prices are inflated because of a lack of competition in the supermarket industry, while alcohol is taxed very heavily by the Government. High tax rates and labour costs may also explain why everything costs so much.

Whatever the cause, you can understand why people get excited about a big sale at their favourite department store.

Summing up all our courage, Sanna and I set our alarm clock early and joined the crush of people at the opening of the sale.

As the video shows, even at just one of the entrances to the department store there were hundreds of people queuing to get in, squeezing together to avoid getting run down by the trams which pass only a few metres away from the doors.

Although not intending to purchase anything, in the end we did take a quick detour to the food section.

Disappointingly we couldn't find the special rye bread which we both like so much, but on the upside bananas were just .99c per kilo.

I went crazy and bought two.

A video of the madness

- Matt Kennedy-Good