A minority of the ocean floor has been mapped at useful resolution. The rest is known only from satellite gravity data, which reveals broad features and misses anything smaller than a large hill.
Why it is hard
Light does not penetrate deep water, so mapping means sonar from ships or submersibles passing directly overhead. The area is enormous and the vessels are slow. Covering it all at current rates would take centuries.
Why it matters
Climate models depend on ocean circulation, which depends on the shape of the floor. Unmapped ridges and channels affect how currents move heat and carbon.
Hazards. Undersea landslides and the faults that generate tsunamis are on the seabed, and predicting them requires knowing where they are.
Resources and cables. Every undersea cable route, every proposed mining site, every offshore energy installation requires survey.
And biology, since most of what lives in the deep ocean has not been described and much of it lives on features nobody has seen.
What is changing
Autonomous vessels that survey without a crew, running for weeks at a time. Coordinated international efforts to pool data that previously sat in separate archives. And processing that turns raw sonar into usable maps automatically rather than by hand.
The mining pressure
Interest in seabed minerals – particularly the metals used in batteries – is accelerating survey in specific areas. That produces maps and it produces conflict, because the same survey that identifies deposits identifies ecosystems that would be destroyed extracting them.
What the maps will show
Nobody knows, which is the point. Every detailed survey to date has found features that were not predicted.