London continues to turn to underground tunnels in efforts to fight overcrowding on its aboveground narrow streets and alleys.

Since ancient times, humans around the world have constructed extensive underground tunnels for everything from burying the dead in catacombs to conducting secret military and government missions. In addition to these purposes, London specifically is continuing to turn to underground tunnels in efforts to fight overcrowding on its aboveground narrow streets and alleys.

150 years ago when England was the wealthiest nation, London opened the world’s first underground railway in response to its rapidly growing population and economy. Between 1801 and 1851, the number of London residents increased from 960,000 to more than 2.5 million.

In recent months, the first tunnel of a new $24 billion railway initiative that people are calling “a wonder of engineering” was finished 40 meters below the city’s surface. Other countries have launched similar record-breaking projects. In 2010, Switzerland unveiled plans for the world’s longest tunnel which will be 35.4 miles long. And China is building hundreds of miles of mostly secret tunnels.

While the soft, dry, and tough clay earth underneath most of London is ideal for building and maintaining tunnels, it’s also made things difficult for builders and engineers who are hard-pressed to find space.