Flanked by two powerful European nations, the English Channel has long been one of the world’s  most important maritime passages. Yet for most of its history, the channel’s rocky shores  and stormy weather made crossing a dangerous prospect. Engineers of the early 1800's  proposed numerous plans for spanning the 33 kilometer gap. Their designs included artificial islands  linked by bridges, submerged tubes suspended  from floating platforms, and an underwater passage more than twice  the length of any existing tunnel. By the end of the century, this last proposal had captured  European imagination. The invention  of the tunnel boring machine and the discovery of a stable layer  of chalk marl below the seabed made this fantastic tunnel  more feasible. But the project’s most urgent obstacles  were ones no engineer could solve. At the time, Britons viewed their geographic isolation  as a strategic advantage, and fears about French invasion  shut down plans for the tunnel. The rise of aerial warfare rendered  these worries obsolete, but new economic concerns  arose to replace them. Finally, 100 years after  the initial excavation, the two countries  reached an agreement— the tunnel would proceed  with private funding. In 1985, a group  of French and British companies invested the modern equivalent  of 14 billion pounds, making the tunnel the most expensive  infrastructure project to date. The design called  for three separate tunnels— one for trains to France,  one for trains to England, and one service tunnel between them. Alongside crossover chambers,  emergency passages, and air ducts, this amounted to over 200 kilometers  of tunnels. In 1988, workers began excavating  from both sides, planning to meet in the middle. Early surveys of the French coast  revealed the site was full of fault lines. These small cracks  let water seep into the rock, so engineers had to develop  waterproof boring machines. The British anticipated drier conditions,  and forged ahead with regular borers. But only months into the work, water  flooded in through undetected fissures. To drill in this wet chalk,  the British had to use grout to seal the cracks  created in the borer’s wake, and even work ahead of the main borer to reinforce the chalk  about to be drilled. With these obstacles behind them,  both teams began drilling at full speed. Boring machines weighing up to 1,300 tons  drilled at nearly 3.5 meters per hour. As they dug, they installed lining rings  to stabilize the tunnel behind them, making way for support wagons  following each machine. Even at top speed,  work had to proceed carefully. The chalk layer followed a winding path  between unstable rock and clay, punctured by over 100 boring holes  made by previous surveyors. Furthermore, both teams had  to constantly check their coordinates to ensure they were on track to meet  within 2 centimeters of each other. To maintain this delicate trajectory, the borers employed  satellite positioning systems, as well as paleontologists  who used excavated fossils to confirm they were at the right depth. During construction,  the project employed over 13,000 people and cost the lives of ten workers. But after two and a half years  of tunneling, the two sides finally made contact. British worker Graham Fagg  emerged on the French side, becoming the first human to cross  the channel by land since the Ice Age. There was still work to be done— from installing crossover chambers  and pumping stations, to laying over a hundred miles of tracks,  cables, and sensors. But on May 6, 1994, an opening ceremony  marked the tunnel’s completion. Full public service began  16 months later, with trains for passengers  and rail shuttles for cars and trucks. Today, the Channel Tunnel services  over 20 million passengers a year, transporting riders across the channel  in just 35 minutes. Unfortunately, not everyone has  the privilege of making this trip legally. Thousands of refugees have tried  to enter Britain through the tunnel in sometimes fatal attempts. These tragedies have transformed  the tunnel’s southern entrance into an ongoing site of conflict. Hopefully, the structure’s history  can serve as a reminder that humanity is at their best  when breaking down barriers.