The ocean. It not only significantly simplifies nutrient delivery but eliminates the geography problem and can utilize the giant hypoxic dead zones in the ocean to aid in organic capture. It also provides a constant mechanism for supplying carbon as atmospheric CO2 dissolves in the oceans that has a positive feedback loop with the climate and houses plankton that is already responsible for the majority of the world's CO2 uptake. There we can store carbon in many forms including microorganisms and seaweed that don't decompose, calcium carbonate in seashells, and so on.
And it doesn't need to be 1000 feet deep, just a few meters worth since we only need to collect an equivalent of a fraction of the CO2 that's stored in peatlands - which cover only about 3% of the Earth's land surface to begin with*. The trick is to sink it in just the right way to keep it from reentering the carbon cycle.
We actually do have a lot of space that's already capturing far more carbon than we ever possibly could ourselves, the organisms filling that space just evolved to maximize survival, not maximize carbon capture to save humanity.
* Edit: actually that's just cold climate peatlands, I haven't seen estimates for tropical peatlands
And it doesn't need to be 1000 feet deep, just a few meters worth since we only need to collect an equivalent of a fraction of the CO2 that's stored in peatlands - which cover only about 3% of the Earth's land surface to begin with*. The trick is to sink it in just the right way to keep it from reentering the carbon cycle.
We actually do have a lot of space that's already capturing far more carbon than we ever possibly could ourselves, the organisms filling that space just evolved to maximize survival, not maximize carbon capture to save humanity.
* Edit: actually that's just cold climate peatlands, I haven't seen estimates for tropical peatlands