The phrase "are mostly below" is correct and usable in written English.
It can be used to describe a situation where the majority of something is situated at a lower level or position compared to a reference point.
Example: "The temperatures in the region are mostly below freezing during the winter months."
Alternatives: "are primarily beneath" or "are mainly under".
Finds from the fossilised forest bed are mostly below beach level; ammonites, shells and reptile fossils may be discovered on the foreshore when the tide is out.
Emissions at low altitude (11 15 km) are mostly below the tropopause leading to a much more limited lifetime of the resultant aerosol.
Starting last year, the watcher and the listener teamed up in hopes of both exploiting and deciphering elephants' low-frequency lexicon -- rumbles that are mostly below the range of human hearing but that travel for miles in hot equatorial air.
Although the Anchor's customers are mostly below the rank of officers, the atmosphere is "trans-atlantic de luxe" something like a second-class smoking room as it is pict red in a steamship folder.
"That said, our bigger sellers are mostly below £4". In Dan's experience at the White Horse, "people don't flinch at anything under £4.70 because in the city you can easily pay this for a pint of lager". Of course, that's in wealthy west London; charge £4.70 a pint in most pubs and you'll knock the regulars off their bar stools.
The most meaningful updates are mostly below the surface — the first full macOS upgrade to do so since 2009's Snow Leopard.
The bowl-shaped bedrock under its ice is mostly below sea level.
Natural subsidence and gas drills are depressing the Frisian territory, which is mostly below sea level.
Investment is mostly below pre-crisis levelsTHE financial crisis began as a credit crunch and its repercussions are still felt.
The response differences calculated using two aerodynamic parameters were mostly below 5%, and regardless of whether or not to consider the influence induced by self-excited force.
COD (chemical oxygen demand), TN, NH4+-N and TP (total phosphorus) in the final effluent were mostly below 30, 15, 5 and 0.5 mg/L, respectively.
