Since I have to do it any is a poor excuse for writing anything, but I have to, at some point in time, actually flesh out this location beyond "it's underwater" for adventure gaming. So without further ado, the history of the Lost City.
Few of the cities in the Old World are as unique as the Lost City. Once known as Atherstone City and perched on Atherstone Island, the Lost City was once a thriving seaport. Atherstone facilitated trade through the sea from Valencia and Havensfield to the southern continent, most notably to Oddessa. Atherstone was somewhat uniquely located in its location separating trade in the Old Sea and trade coming down the coast from the White Sea or up from the south.
Atherstone, similarly to the other islands in the old sea, was originally colonized by the vikings. In early times, the vikings controlled the vast majority of the waterways since there were so few ships. The first crusade, led by Thomorn Dragonsbane, cleared the vikings from the waterways and pushed them back onto the southern continent. The first crusade would see the creation of Valencia and Havenfield, as well as the founding of Atherstone. At that time, Atherstone functioned as a restocking station for the crusade. Thomorn went on to conquer three of the four islands in the Old Sea and would establish colonies on each of them to facilitate his eventual crossing to the southern continent. Atherstone was a key part of the crossing and made the crossing possible. Though Thormon was eventually defeated by the vikings he decreased there numbers sufficiently that Atherstone was rarely threatened by them again.
Atherstone stayed as a small refueling station for the navy until the founding of Oddessa, which greatly increased trade through the Old Sea. Similarly, metals and machinery became more common place as the Fortview Furnaces grew and the products from Fortview were shipped down the coast. Havenfield became a bustling city and as trade from Havenfield increased, so to did the number of ships making port in Atherstone. Metals and exotic foods were shipped from Havenfield and Valnecia through to Riga and from Riga into the Inner Sea. As trade increased Atherstone changed into a bustling trade city, fuelled by the ships making port there for supplies.
Havenfields proximity to Waltham would eventually lead it to adopt the technologies more common in the west. A university was founded at Havensfield to study the items brought up from Oddessa and to mount expeditions into the south. Soon a university was founded in Atherstone as well in order to support both these pursuits. While Havensfield's university would slowly die of stagnation and lack of trade during the third crusade, Atherston's University flourished studying the diverse sea life and exotic plants and animals brought by the servants of the Night Lords fleeing the church and by expeditions into the Girga desert.
Meanwhile Valencia would go on to become a major city of the Church, one of the few western cities to still have a priestly leader. Valencia became a holy city, frequented by thousands of pilgrims to see the fabled walls of Valencia, a massive barrier commissioned by the Pope himself to protect Valencia from sinking into the sea. Valencia's expansion had brought it perilously close to destruction and the church's decision to save it made it a holy city.
At the beginning of the fourth crusade, it became obvious that Valencia and Havensfield would become the natural choke points of the attack. The Fortview Mountains prevented an attack at the heart of the west and an attack around the mountains would be difficult to mount in the face of stiff resistance and difficult weather and terrain. Valencia became a gathering point for Cruice's army.
About this time, the Free Cities elected the Sunset Kid to lead an army against Valencia. Knowing Valencia's walls would be unable to protect it against airborn attack, the Kid staged a large force of airships at Atherstone. Meanwhile, his ground troops moved to defend Havensfield from any ranging forces.
Unfortunately, Cruice discovered the Sunset Kid's plan, and in a bid to save his forces and Valencia he commissioned the vatican to sink the isle of Atherstone. The pope and his councellors gathered there might and sank the island beneath the waves. Atherstone, home to both a university and several powerful mages who were part of the war effort, was saved by a massive bubble spell which protected the city as it sank. The spell is similar to that which protects Valencia and at the time the mages of Atherstone's university were working on such a spell to protect the city from vicious ocean storms. In a stroke of luck, the spell was largely prepared ahead of time and while the mages were not able to repel the church's power they were able to protect the city. The spell filters air from the water, allowing air to reach the city and stabilizes the pressure in the city, allowing humans to survive in it. The casting of the spell cost many mages there lives, a sacrifice still felt by the Free Cities due to the lack of properly trained war mages.
The destruction of the island caused massive tidal waves across the Old Sea. Valencia's walls held it safe, but the Sunset Kid's fleet was not so lucky. The fleet was largely destroyed and most of the airships, tethered to the island, were destroyed as well, sunk beneath the waves as there tethers sank or destroyed by the church's magic. The magicians tasked with protecting the fleet dedicated there energy to saving the city and as a result the Sunset Kid's army was reduced to a fraction of its original size. Many of the war mages the Sunset Kid has brought with him died saving the city as well, resulting in a serious reduction of the magical power of the Free Cities. Fortunately, the amount of power it took the church to destroy the island largely neutered the Vatican as well, and such power has yet to be displayed again during the war.
Now, Atherstone, known as the Lost City, functions as a sea floor base for the Free Cities. The city has continued to grow as deep sea mining operations have since set up in it. Largely protected from the church, the Lost City is a powerful stronghold but also a poor place to amass an army. Few resources can be gathered besides gasses from the ocean floor, and the majority of these resources are needed to sustain the city. Vast kelp fields, maintained by automatons and humans in pressure suits, keep the city fed. The lower classes feed on kelp and fish while the upper classes generally eat more exotic foods such as beef and chicken. The Lost City has certainly lost its former glory, but its unique location has led to many discoveries that would not have come about otherwise.