Short answers, pulled from the story.
The Mannerheim Line was a defensive fortification built by Finland across the Karelian Isthmus to protect against a Soviet invasion. Its purpose was to delay rather than repel an attack, using natural terrain, trenches, and obstacles alongside concrete bunkers.
The line took shape through multiple designers over two decades. Lieutenant Colonel Johan Fabritius played a central role in both construction phases, working alongside French military commission member Major J. Gros-Coissy. The overall siting was set by Finnish Chief of Staff Major General Oscar Enckell in 1919.
Far weaker in concrete terms. Finland built 101 concrete bunkers across the entire line, while an equivalent stretch of the Maginot Line held 5,800 bunkers linked by underground railways. The total concrete used in the Mannerheim Line, 14,520 cubic meters, was slightly less than the amount used in the Helsinki Opera House.
Construction occurred in two phases: 1920-1924 and 1932-1939. The line was still incomplete when the Winter War began in November 1939.
Soviet intelligence gathered information through Finnish Communist Party networks, NKVD agents, and individual spies including photographer Vilho Pentikäinen in 1933 and Simo Haukka in 1935. By 1938 Soviet intelligence had published a numbered top-secret photobook of Finnish fortifications, and in September 1939 a German military attache in Helsinki, General Arniké, handed a detailed map of the isthmus defenses to Soviet officials in Moscow.
Soviet propaganda spread that claim during the Winter War to explain the Red Army's slow progress against Finnish defenses. Finnish propaganda also exaggerated the line's strength to boost national morale. The myth entered official Soviet war history and some western sources, even though the Mannerheim Line was a flexible field fortification with far fewer concrete structures than the Maginot system.