Entrance to the harbour of Helsingfors, Finland partly obstructed according to the Weekly Intelligence Summary No. 1 dated 8 January 1921

An item reported that in the outer harbour of Helsingfors [Helsinki], Finland four mines exploded after striking rocks after the last storm (of 3^rd January?). Until now no mine was sighted in the fairway. Large ships were to use the outer channel towards Arans Grund and from there using the ordinary channel leading to Helsingfors. 

Source

The National Archives, Kew Gardens, England CAB/24/118/41