Just at the former Finnish-Russian border at Sestroretsk (finnish Systerbäck) lies a large bunker that was part of the Soviet defense line that ran along the finnish-russian border on Karelian isthmus. The Bunker and the lin defense line began to be built during the thirties to defend an attack against Leningrad. At this time Leningrad was only about twenty kilometres from the border and thus extremely exposed. It was not necessarily a Finnish attack that the russians feared, but more that German troops would have access to Finnish territory to attack the Soviet Union from Finland.

Current status: Partly preserved/demolished with information board (2021).

Location: 60°06'49.3"N 29°58'16.8"E

Get there: Car

My comment:

The Bunker is a museum with limited opening hours, but all materials and equipment found by war archaeologists and military interested is exhibited outdoor and can be visited 24/7. 

Follow up in books: Trotter, R William: The Russo-Finnish Winter War of 1939-1940 (2010).