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Why locals love it
Mustavuori's Base is extensive military outpost structures in the Mustavuori's Nature Reservation area in Helsinki. Man-made outposts are apart of a bigger defense lines, that has been made in the First World War Period by the Russians. More about history : http://www.rky.fi/read/asp/r_kohde_det.aspx?KOHDE_ID=1570
Why you should visit it
Trenches are strategically scattered around the Mustavuori. It is a extensive military base, that is apart of a crescent shaped outpost structure around the Helsinki Metropolitan area. On a vast area there's lot of ruins to be discovered. Make sure that you take consideration of dangers before heading to the sites.
Terrain is really rocky, good shoes are required. Be careful on the site, trenches are some places pretty deep, danger to fall is possible.