Below sea level is a real number
Elevation returns a real negative number over open ocean, the same field as anywhere else.
The ocean floor is still a coordinate.
/elevation samples a grid and returns whatever is there. On land that is usually positive. In the middle of the Pacific, nowhere near a coastline, it is thousands of meters negative.
GET /elevation?lat=0&lon=-140
{ "latitude": 0, "longitude": -140, "elevation": -4234, "elevation_ft": -13891, "resolution": 30 }
No separate flag, no special-cased field. elevation and elevation_ft go negative over open water the same way they go positive on a mountain.
Place fields are the only null
/point returns that same elevation value at the identical coordinate, alongside country, state, and district, and those three come back null. Elevation resolves everywhere. Place fields only resolve where there is a place.
GET /point?lat=0&lon=-140
{ "elevation": -4234, "elevation_ft": -13891, "country": null, "state": null, "district": null }
Try it
Call /elevation on any point in open ocean and watch the number keep dropping toward a trench. Nothing stops it. There's no floor field to catch it.