I found the normally flooded cave frozen solid. This meant I could walk right in.

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It was impossible not to be at least a little bit curious about Mustikkamäki (”Blueberry Hill”) Cave. The stories told of a flooded cave in East Helsinki’s Kontula district that was deceptively easy to get to but notoriously difficult to explore; its watery depths penetrable by diving or boating alone; the unusual resourcefulness (or foolhardiness?) required from those hoping to do just that.

So when I set to find the place a couple of weeks ago on a beautiful but cold winter’s day, I had no idea what to expect. Would I even be able to enter the cave? Being out on my own with no special equipment beyond my winter boots, camera gear and flashlight, I thought it best to play it by ear and enjoy whatever shape the outing might take.

Needle in a haystack

My convictions were put to the test as soon as I reached the edge of Mustikkamäki Hill, a pristine patch of woodland landlocked inside a wealthy residential area and one of the dozens of sites in and around Helsinki belonging to Kreport Sveaborg: the landward defence zone of the Sveaborg Fortress built by the Russian Imperial Army between 1914 and 1918.

It had been snowing heavily in Helsinki and all of Finland for the past weeks, which made the landscape virtually unreadable in places. I had only a vague idea of where I might expect to find the entrance to the cave – that is, if it wasn’t obscured by thick layers of fluffy white snow.

Oh well, I thought looking at the soft whiteness glimmering in the afternoon sun. At least it’ll be a nice walk.

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Songs from the trenches

I walked along one of the paths that could still be made out from under the snow. Despite its relatively small size, the hill felt peaceful and somehow secluded that day, like a little slice of pre-modern Finland that city planners forgot. I crossed a couple of riverbeds that I soon realized were not riverbeds at all, but trenches belonging to the defence zone. At least I was in the right area!

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Today some of these trenches are taken over by weeds and garbage while others are cleaned out semi-regularly, but in the snow there was no visible difference. I followed the narrow hollows towards the highest spot of the hill, figuring it was around there that I was most likely to find the entrance to the cave. After all, it was built to keep the enemy at bay.

I wasn’t wrong. Soon enough, I was standing on a sheltered spot of land between a couple of large boulders and heaps of snow. In front of me was a pitch-black doorway leading directly into the hill.

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The German threat

The caves, bunkers, gun batteries, trenches and other structures that make up Krepost Sveaborg are products of the First World War. When the bloodshed broke out in Europe, Imperial Russia became concerned with the possibility of an enemy, mainly Germany, attacking St. Petersburg via Helsinki, the then-capital of the Grand Duchy of Finland.

Between 1914 and 1918, a landward defence zone was erected in and around Helsinki to support the already-existing Sveaborg Naval Fortress, which consisted of six islands in the Gulf of Finland (and still does). Somewhat ironically, the original purpose of the Swedish-built fortress had been to stop Russian expansion some 150 years earlier.


Archival image of Russian soldiers in Helsinki. Source: Helsinki City Museum, photo by Ivan Timirjasev, 1914

The sheer man power that went into Krepost Sveaborg’s construction was enormous. Extra hands were brought in from all over Finland and Russia; many of the foreign workers were of Tatar, Kyrgyz or other Russian minority origin. Not everybody came voluntarily – the Imperial Army did not shy away from forced recruitments – but even then the gig paid, and conflict was avoided.

Mustikkamäki Hill was one of the single largest sites belonging to the newly built defence zone.

Interactive map of sites connected with Kreport Sveaborg. Source: Yle

Into the hill

Approaching the cave’s entrance, the first thing I noticed were dozens of white, candle-shaped things on its floor. Like a good 1980’s kid with vivid memories of the Great Satanic Panic of her childhood years, my first thought was that I was looking at the set-up of some dark nocturnal ritual that had taken place inside the cave not too long ago. My second thought was that I was being stupid.

Upon closer inspection, the white objects turned out to be not candles but ice spikes popping out of the cave’s black icy floor like mushrooms in the dark. The normally flooded cave was frozen solid. I’d of course seen pointy-ended ice spikes before, but these blunt formations were completely new and fascinating to me. The subzero temperature inside the cave meant one additional thing: I could walk right in.

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Passing through a small entry room to the main tunnel, I turned on my mighty Maglite. The darkness around me gained form. I could make out the other end of the tunnel in the distance – it looked to be some 30 meters away from where I was standing – where it took a sharp turn to the right. What lay beyond I could not see.

I figured I might be able to walk the entire length of the tunnel, but decided against it. I knew the water to be as deep as 1.5 to 2 meters further inside the cave and there was simply no guarantee that the ice would hold. I settled for gazing into the abyss and trying to get a few passable shots of the cave under the challenging weather conditions, listening closely to the eerie humming sound emanating from somewhere far out of my reach.

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Later I found out that I had made the right choice not to venture deeper. In his book Hämäräperäisiä tutkimusmatkoja (”Shady explorations”), which I will return to at the end of this post, artist Jussi Kivi explains how warm groundwater in the Mustikkamäki area prevents cave from freezing thoroughly even in the dead of the winter. While the front was safe, the depths most certainly would not have been. Let this serve as a warning to any reader inspired to try their luck…

Ghosts of an attack that never was

Krepost Sveaborg was never used for its intended purpose. The only action the defence zone ever saw was a 1918 battle between the Finnish Reds and German troops closing in from the North. In absence of attack from the sea, and with Mother Russia thrown into its own revolution of 1917, the fortifications soon became obsolete. Finland finally gained its independence.

During the Second World War Mustikkamäki Cave served as a small ammo factory and in the post-war period as a storage space.

Since 1971, all structures belonging to Kreport Sveaborg are protected under the Antiquities Act, which stipulates that they cannot be filled, dug or otherwise destroyed without permission from the National Board of Antiquities.

And so the derelict structures sit silently in our own suburban backyards, while life goes on around them. Their obvious lack of glamour is enough to repel everybody except the occasional Sunday stroller, history buff, intrepid explorer and young hooligan.


Archival image of German soldiers at Ilmala weather station located some 12 kilometers from Mustikkamäki. Source: Noviisikilta

Acts of unusual heroism

To explore Mustikkamäki Cave in its entirety one needs more than a good pair of wellies. In Hämäräperäisiä tutkimusmatkoja, Jussi Kivi recounts the story of how he managed to reach the other end of the cave with the help of some sturdy chest waders (for himself) and a small inflatable boat (for his camera gear). It was an unusually dry summer in Finland and the water inside the cave was at its lowest in living memory.

Kivi’s effort was rewarded with a first-hand, unadultered look at the T-shaped structure with two empty rooms at its end. I don’t know about you, but to me the journey, and the dedication required to make it happen, sound pretty amazing in themselves.

Kivi’s excellent book is unfortunately only available in Finnish, but English-speaking cave fans in Finland may pick up First World War Fortifications in and around Helsinki: Guide to the Landward Defence Zone of the Sveaborg Fortress by archeologist John Lagestedt. I’ve only just acquired my own copy, but I’m already planning to use it a lot in the upcoming spring and summer.

As for a possible return to Mustikkamäki, I’m not ruling anything out. I always did wonder how I’d look in chest waders…

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First World War Fortifications in and around Helsinki. Guide to the Landward Defence Zone of the Sveaborg Fortress, John Lagerstedt (Helsinki City Museum 2015)
Hämäräperäisiä tutkimusmatkoja, Jussi Kivi (Into Kustannus 2015)


Arkeologi tekee opaskirjaa linnotteista, joiden ympärillä asuu iso joukko helsinkiläisiä, Helsingin Sanomat (23/11/2013)
Helsinkiä kiertää 1. maailmansodan linnoitusketju – katso kartta!, Yle (29/9/2014)
Krepost Sveaborg, Wikipedia


  1. Kiintoisaa nähdä kuvia tästä hattivattiluolasta… Matti Yrjänä Joensuun “Harjunpää ja poliisin poika” -romaanissahan päähenkilöt pakenivat tuonne poliiseilta: soutivat lautalla luolan perukoille ja leiriytyivät sinne teltassa.

    • Toden totta! Luin Harjunpää-yhteydestä ensi kertaa vasta Jussi Kiven kirjasta, mutta ilahduin siitä kovasti siksikin, että satuin lapsena asumaan vuosikausia Matti Yrjänä Joensuun naapurissa. Ajatus yläkerran joviaalista, nyt toki jo edesmenneestä herrasmiespoliisista tutkimassa Mustikkamäkeä kirjaa varten on kerrassaan mukava.

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