The deep sea holds more living space than every other habitat combined, and the fraction of it that has been observed directly is very small. The reasons are practical and they compound.
The physical barrier
Pressure increases steadily with depth, reaching values at the abyssal plain that crush ordinary equipment and require thick-walled housings for any instrument sent down.
Those housings are heavy and expensive, and every additional camera, light or sampler must be independently rated. Complexity therefore rises far faster than depth does.
Sunlight is absent below a few hundred metres, so observation requires carrying light, which consumes power and alters the behaviour of animals adapted to permanent darkness.
Ship time is the limiting resource
Reaching a deep-sea site requires a research vessel, and vessel operation costs are substantial per day regardless of what the science accomplishes.
Transit consumes much of a cruise, since deep sites are often far from port, and weather can remove working days from a schedule fixed months earlier.
The consequence is that sampling concentrates near a small number of well-studied locations, and vast areas remain unvisited because no expedition has ever passed over them.
Why sampling misrepresents the animals
Traditional sampling uses trawls and grabs, which recover material but destroy delicate organisms in the process. Gelatinous animals frequently arrive as fragments.
Decompression and warming during ascent kill most deep animals and distort their bodies, so specimens described from nets differ from the same species observed alive.
Remotely operated vehicles have corrected part of this, revealing abundant soft-bodied life that trawl surveys had systematically underestimated for decades.
The taxonomic backlog
Describing a new species formally requires comparison against existing descriptions, specialist knowledge of the group and publication, and specialists in deep-sea groups are few.
Material therefore accumulates in museum collections awaiting description, sometimes for many years, and undescribed species cannot be assessed for conservation status.
Genetic sequencing has partly filled the gap by identifying distinct lineages quickly, though a sequence without a formal description leaves the species effectively invisible to policy.
Why it matters now
Interest in seabed mineral extraction has moved ahead of the biological knowledge needed to judge its consequences, particularly on abyssal plains and seamounts.
Baseline surveys commissioned for that purpose have produced a substantial share of recent deep-sea discovery, which places much of the knowledge inside a commercial process.
Recovery rates compound the problem, since deep-sea animals grow slowly and reproduce late, so disturbance persists on timescales measured in decades or longer.