Visiting mayors from Ukrainian cities took pictures inside an enormous cave carved in the bedrock below the Finnish capital, taking in the size and the possibilities of a space that can accommodate 6,000 people.

They are among around 800 foreign delegations who have visited Helsinki's Merihaka civil shelter - the biggest of the country's dual-use shelters and now a major shop window for Finnish companies looking to export to buyers concerned about the war in Ukraine and, increasingly, the conflict with Iran.

At 71,000 cubic metres, the shelter is roughly the volume of a seven-storey office building. It was built in 2003 and has sports pitches, a gym and a children's playground 25 metres (80 ft) underground that are in use daily.

Should it be needed in an emergency, it can be converted with stores of bunk beds, water tanks and portable toilets within 72 hours.

GROWTH POTENTIAL

As well as being a testament to Finland's painful history with neighbouring Russia during World War Two, building such shelters remains mandatory underneath residential and commercial buildings of a certain size.

With that requirement, Finnish companies are skilled in their construction and upkeep - such as fitting and maintaining radiation-proof doors, ventilation equipment and emergency power, as well as communications and sewerage networks.

Politicians, administrators and businesses from overseas, such as oil giant Saudi Aramco, have all sought information or visits, authorities and companies involved said.

Resilience Center Finland, an exports body established in March, said Finland's security and defence exports stood at tens of billions of euros, with shelter sales amounting to some dozen million euros, with significant growth potential.

"Within two years we won't need to compete fiercely with our peers over getting a gig. Rather, it will very soon be a question of capacity running out," Ilkka Kivisaari, CEO of Finnish-Swiss-owned Verona Shelters Group, said, citing high demand in countries such as Poland and Germany and high interest from the Middle East.

INTEREST FROM GULF

Merihaka is among 48 big and 5,500 smaller shelters in Helsinki - among 50,500 constructed around the country, the legacy of a Soviet invasion attempt during World War Two.

At the entrance of another shelter in the city, which can house 3,800 people, Juha Simola, the CEO of Finnish Temet Group arrived straight from the Czech Republic after an export promotion trip led by Finland's President Alexander Stubb.

He was showing shelter technology to visitors from Saudi Aramco, the world's top oil exporter, and said he had also had interest from elsewhere in the Gulf during the war in Iran.

"There was a quite big hit in Abu Dhabi and I got a phone call from there that please come quickly," Simola told Reuters, without giving further details.

He said his company was building a factory in the United Arab Emirates, which he said was planning to construct hundreds of shelters.

Temet, which has been in the business for 70 years, hopes to book 80% of its sales from exports over the coming years, Simola said.

While Temet and Verona are Finland's biggest firms operating in shelter solutions, there are several smaller companies specialising in specific areas such as blast doors and communications.