Image: Sergi Reboredo / AOP

Helsinki is possibly the only city in the world with an official subterranean zoning plan and at present the city's zoning office is updating the last plan from 2011.

Maintenance tunnels crisscross the urban area at a depth of around 80 metres below the surface, but zoning for construction and use extends down to as much as 100 metres. There are vast spaces carved out of the bedrock, totalling close to 10 million cubic metres.

Image: Yle Uutisgrafiikka

Here are a few examples of some of the better- and lesser-known sites under the city.

1. Few of the people enjoying the summer sun in the Esplanadi Park in central Helsinki are likely to realise that they are sitting on top of an underground lake. Forty metres below the park is a water reservoir 40 metres wide and 80 metres long, owned by the city's power utility Helsinki Energy - Helen. The bottom of the reservoir is 100 metres straight down.

2. The same company has a second reservoir below the city's Pasila district. Twenty metres on one side and more than 30 metres deep, like the one under the centre, it is a cold water storage facility for a district cooling system.

Image: Antti Kolppo / Yle

3. Viikinmäki water purification plant. Mostly underground, the plant processes the waste water generated by 800,000 people in Helsinki, central and eastern Vantaa, Kerava, Tuusula, Järvenpää, and Sipoo. The purified water is routed through an 8 km tunnel to the tip of Hernesaari that empties 20 metres below sea level.

4. Katri Vala district heating and cooling plant. Located in a cavern excavated under Katri Valan Park this plant began operations in 2006. It is the world's largest heat pump facility using the same process to generate both heating and cooling.

5. The underground Itäkeskus swimming hall opened in 1993. Constructed in two underground levels, it can accommodate around 1,000 patrons at a time.

Image: Merja Huovinen / Helsingin kaupunki

6. Just to the west of the Olympic stadium an underground sports facility is currently being carved out of the bedrock. Forty meters long and nine metres high, when completed, the space will be big enough to fit in Finlandia Hall four times over.

Metro lines and ghost stations

7. Excavation of a station for a planned light underground railway was carried out under the Munkkivuori shopping centre in the 1960s. The plan was abandoned and construction suspended. The incomplete station is still there.

8. Beneath the Kampi metro station is another of nearly the same size that was excavated in the 1970s in anticipation of the opening of metro line linking to the present route. This stop also remains unused.

9. There is a partially-completed station hall parallel to the Hakaniemi metro station that was excavated for another line that was never launched. If and when a rail loop under the city is ever built, this station is likely to be finished.

10. The central railway station has a ghost metro station built in the 1960s. However, it was decided to construct the metro at a lower level and this station was bypassed and replaced by a deeper one. Directly under Kaivokatu, it's been used for a variety of purposes such as housing squash courts.

11. A fifth ghost station is currently being built in central Pasila under the site of a new shopping mall. It will lay dormant waiting for a new metro line planned for sometime in the 2030's.

Present metro stations

1.–6. Helsinki's present metro runs by tunnel between Ruoholahti and Sörnäinen at an average depth of around 25 metres. The deepest platforms are to be found at the Kampi station where they are 30 metres below the surface.