Various proposals have been floated for producing new solid land on the ocean. This can be more ambitious than contemporary seasteading, which typically involves smaller floating structures and cities.

This project has various immediately legible advantages such as the absence of building codes and access to shipping lanes. It's also relatively inexpensive given anything like current launch costs in comparison to space colonies and other planets. It may also have ecological benefits such as reducing global warming due to higher albedo than ocean water. The continent may be large enough to be meaningfully classified as a state or country.

There are likely benefits to energy production mechanisms such as nuclear power (mostly regulatory advantages), proximity to offshore wind and solar, oceanic methane extraction, tidal/wave energy, and so on. Various metals (such as magnesium) are relatively abundant in the ocean and could be extracted by known processes given energy resources, and fish and algae could be farmed as biomass or fertilizer for crops grown on the land.

One such proposal is to form the floating land system from ice, adding insulating layers and minimal cost cooling. Additional reinforcement could be added to the ice, creating a form of pykrete. The insulation efficiencies in terms of materials needed are improved by the square-cube law.

https://transhumanaxiology.substack.com/p/ice-the-penultimate-frontier

This could be made indistinguishable from land, with dirt (synthetic or imported) added to agricultural areas, and the buildings on top could be ordinary non-floating buildings, potentially metropolis-scale vertical construction.

This question is resolved by any large scale structure (100 square kilometers in area or greater) completed by the year 2050. It also counts if the purpose is not habitation, e.g. it could be for hurricane management.

Large natural icebergs (some of which have greater surface areas than this) do not count, but if one were to be insulated and made capable of lasting 10 years or more in warm waters for an economically useful purpose such as industry, hurricane mitigation, habitation, etc. they would.