Imagine a city at the bottom of the North Atlantic Ocean, in an area known as the Mid-Atlantic Ridge, about 2000m deep under the sea, with towering buildings made of glass and steel that glitter in the ocean's depths. This is the world of Rapture, the underwater city featured in the Bioshock games. Rapture was designed as a utopian society for the world's best and brightest minds, far away from the chaos and politics of the surface world.

To withstand the immense pressure of the ocean depths, the builders of Rapture utilized advanced building materials such as a special type of concrete known as "Rapturite," which was reinforced with an advanced metal alloy. They also employed specialized construction techniques and machinery to ensure that the buildings and structures of Rapture would be able to withstand the harsh underwater environment. The materials were submerged to the ocean floor via a giant submersible platform nicknamed "The Sinker." Deep-sea welders and mechanics created a foundation for the city by sinking pilings and girders deep into the rock and silt. Pre-fabricated buildings with frames of aluminium were assembled near the surface, submerged, and lowered using lunette rings, and anchored into the foundations, thus creating the Art Deco metropolis.

Rapture's architecture was a marvel of engineering, with towering buildings that reached up towards the surface. The city was also self-sufficient, with its own farms, factories, and even an art museum. It was powered by geothermal energy and had its own transportation system, with trains and a fleet of submersibles that connected the different districts.

However, despite its idyllic appearance, Rapture was not without its problems. The city's isolation from the rest of the world meant that it was vulnerable to internal conflicts and power struggles, which eventually led to its downfall. The city fell into disrepair and became overrun by mutants and other dangers.

While the fictional city of Rapture may seem like a distant dream, the idea of underwater cities is not as far-fetched as it may seem.

Building an underwater city presents a significant challenge due to the immense pressure of the water at great depths. At a Rapture-depth of 2000 meters, the water pressure is approximately 200 times greater than at the surface, which can exert a tremendous amount of force on structures and materials. To put this into perspective, the pressure at sea level is only about 14.7 pounds per square inch (psi), while at 2000 meters deep, the pressure is approximately 3000 psi. This pressure can cause structural damage to buildings and equipment and can also cause health problems for inhabitants.

Other challenges include the need for self-sustaining energy, water filtration, and waste management systems, as well as transportation infrastructure.

An underwater city would need to be self-sufficient in terms of energy, water, and waste management. This means developing sustainable sources of energy, such as geothermal or hydroelectric power, and designing water filtration and waste management systems that can operate effectively in an underwater environment.

It would need to have a transportation infrastructure that can move people and goods between different parts of the city and connect it to the surface world. This could involve developing submersible vehicles or underwater trains that can travel long distances at high speeds.

The city would also need to have robust safety and emergency response systems in place to protect its inhabitants in case of accidents or disasters. This could involve developing protocols for evacuating the city in case of emergencies, as well as designing safety features such as emergency airlocks and pressure-release systems.

A different kind of impact would be mitigating the environmental impact, both on the surrounding marine ecosystem and on the wider ocean. It would be important to design the city in a way that minimizes its impact on the environment and to monitor its effects over time. This could involve measures such as using renewable energy sources, developing sustainable aquaculture practices, and monitoring water quality and biodiversity.

Despite these, there are already some examples of underwater structures in existence, such as oil platforms and underwater research centres:

Aquarius Reef Base is an underwater research laboratory located off the coast of Key Largo in Florida. Operated by Florida International University, the facility allows researchers to live and work underwater for extended periods of time. The lab sits about 19m underwater on a sandy patch near Conch Reef, about nine miles south of Key Largo. The size of the facility is roughly equivalent to that of a school bus and can usually accommodate around 7 people at a time. Despite its small size, Aquarius Reef Base has served as an important research tool for marine biologists and other scientists interested in studying underwater environments and ecosystems.

Deep-sea oil rigs are often located in waters that are several thousand feet deep, presenting significant engineering and logistical challenges. These rigs are designed to withstand harsh ocean environments, including strong currents, extreme temperatures, and corrosive seawater. The Perdido oil platform in the Gulf of Mexico is a prime example of a deep-sea oil rig, with a height of nearly 600 meters from the seafloor to the top of the drilling rig.

One of the most famous underwater tunnels is the Channel Tunnel, also known as the "Chunnel," which connects England and France. This tunnel is approximately 50.45 kilometers long and sits 75 meters under the surface of the English Channel. The Seikan Tunnel in Japan is another impressive example of a submerged tunnel. This tunnel is 53.85 kilometers long and reaches a depth of 240 meters below the surface of the Tsugaru Strait.

Underwater hotels offer a unique experience for travelers seeking an adventurous and luxurious stay. One such hotel is the Conrad Maldives Rangali Island, which is located in the Indian Ocean and features an underwater restaurant and suites with breathtaking views of the ocean. The hotel's underwater restaurant, Ithaa, is located 5m below sea level and offers a 180-degree panoramic view of the coral reef and marine life. The hotel also has a suite called the Muraka, which is located 16 feet below sea level and features a master bedroom, living room, and bathroom with floor-to-ceiling glass walls offering an unobstructed view of the ocean.

In conclusion, while building an underwater city may seem like a daunting task, it's not completely submerged in impossibility. With advances in technology and innovative thinking, it's possible to dive into the depths and create a submerged utopia that doesn't sink like a stone. So let's not water down our ambitions and keep our heads above the surface, as we explore the possibilities of creating a new wave of underwater architecture.