Mt Ryan

* See also Sea_Mount
* See also Depth
* See also Lighthouse
* See also Lighthouse_Island
* See also Geography
Under the Sea ...
The Ancient Volcano Rapture is built on ("Mt Ryan", as it was named ) was formed from a moving Hot Spot of volcanic activity ( Similar to the one that created the Hawaiian Island chain ). Because it moved, it built a chain of submerged islands (sea mounts) somewhat near Rapture. Now whether this actually has any value is unclear, as they would have the same resources as the Mt Ryan vicinity, which already has sufficient space for development. Many of the other Sea Mounts may not have been as large or as tall ( to fit the tall impressive buildings Ryan wanted ) OR were deeper, so were skipped over when Rapture's site was chosen.
--- --- --- --- ---
Rapture is Built Within a "Caldera" https://www.merriam-webster.com/dictionary/caldera
A hollowed space within the center of an ancient volcano, which is sited so to be well above the ambient Ocean Floor ( which is at least 3000 feet deep at that point in the Ocean ).
--- --- ---
Mount Ryan :
* The large rock mass that sticks up to north of city ( blocking icebergs as it sticks up quite close to the Ocean surface )
* Lighthouse location ( a city 'big' project proposes making that 120 ft lighthouse a bit less obvious )
* Site of City Air Intakes (air renewal system) piped to the rest of the city.
* Shallow Ocean area (100 ft depth) a good sized flat expanse ( and mass of solid rock ) :
** Allows seaweed growth because light penetrates ( site of semi-automated seaweed Farms/plantations )
** Lower pressure differential - less than most of the rest of the city ( useful if venting gases/vapors )
** Simpler 'diving' equipment required ( not the 'Hardsuits' required for the 800 ft depths of the main city areas/foundations )
* Light Penetration of Ocean :
** Only 73 % of the surface light reaches a depth of 1 centimeter (less than a half inch)
** Only 44.5 % of the surface light reaches a depth of 1 meter (3.3 feet)
** 22.2 % of the surface light reaches a depth of 10 meters (33 feet)
** 0.53 % of the surface light reaches a depth of 100 meters (330 feet)
* Solid rock, good for building 'cheap' Tunnels and caverns for City 'space'/storage :
** Rock is self-supporting, requires minimal reinforcement, no pressure walls, and largely no windows.
** Used by manufacturing/industry/warehousing/farms/transportation that need volume.
** Tunnel boring machine was used to create alot of space for future city expansion.
* Fresh Water reservoirs located at a level well above most of City, so that gravity used to pressurize the cities water system.
* Site of the city garbage incinerators (electric furnace) and 'The Dump' ( destination of the city garbage disposal/transport system using the trams ).
* Location of other industrial facilities that require alot of air ( smelting, refining, cement making, etc... ) 
--- --- ---
The Responsible Worldbuilding Required Power For Rapture :
The volcanic location allowed for Geothermal sources of Power and Heating for Rapture.
--- --- ---
--- --- --- --- ---
.
.