The Métro is mostly underground ( of ). Above-ground sections consist of elevated railway viaducts within Paris (on Lines 1, 2, 5 and 6) and the suburban ends of Lines 1, 5, 8, and 13. The tunnels are relatively close to the surface due to the variable nature of the terrain, which complicates deep digging; exceptions include parts of Line 12 under the hill of Montmartre and line 2 under Ménilmontant. The tunnels follow the twists and turns of the streets above. During construction in 1900, a minimum radius of curvature of just 75 metres was imposed, but even this low standard was not adhered to at Bastille and Notre-Dame-de-Lorette.