Tunnels can be built below ground level roads at depths of -12.5m or below. Descent ramps must be reasonable length to avoid traffic congestion. Roads can be built at different elevations within tunnel sections. Maximum tunnel depth is capped at -50m. Tunnels can overlap with 8.75m spacing between levels
Trench is a deep excavation narrower than its length. Created by erosion or tectonic plate movement. Used for underground utilities installation and construction. Archaeological trench method excavates ancient ruins. Geotechnical engineering locates faults and soil properties
Mr Wilson receives a job advertisement for The Red-Headed League. League offers four pounds weekly for red-headed men over 21. Wilson's assistant Vincent Spaulding works for half-pay
Road tunnel crossing Bosphorus Strait in Istanbul, Turkey. Opened December 2016, connecting Kumkapı to Kadıköy. Maximum depth of −106 m, journey takes about 5 minutes. Daily toll: ₺156 for cars, ₺234 for minibuses, ₺60.80 for motorcycles
First proposed in 1802 by Albert Mathieu-Favier. Military concerns in 1919 and 1920s led to cancellation of initial plans. Project cancelled in 1975 due to UK-France economic uncertainty
Tbilisi metro opened in 1966 with 5 stations, now has 23 stations totaling 26.4 km. System carries about 600,000 people daily, serving 97 million passengers annually. Construction began in 1952, with deepest station Rustaveli at 60 meters underground