Measuring the depth of the ocean is far more difficult than mapping land. While satellites can measure the ocean surface with radar altimetry, the deep seafloor remains largely unexplored. According to the Nippon Foundation-GEBCO Seabed 2030 Project, only about 26% of the global ocean floor had been mapped in high resolution as of 2024. The rest is either poorly mapped or completely unknown.
One reason is that sound waves, not light, are used to measure depth. Ships use multibeam echosounders to send acoustic pulses to the seafloor and time their return. This method is accurate but slow: a single ship can survey only a small area per day. Covering the entire ocean would require many ships working for years. Satellite altimetry can infer large-scale seafloor features from variations in gravity, but it cannot resolve fine details like seamounts or trenches.
Another challenge is the extreme environment. The deepest point, the Challenger Deep in the Mariana Trench, reaches about 10,935 meters (35,876 feet) below sea level. Pressure at that depth exceeds 1,000 times atmospheric pressure, and temperatures near freezing. Instruments must withstand these conditions, and deploying them is expensive. As a result, more than 80% of the ocean remains unmapped, unobserved, and unexplored, according to the National Oceanic and Atmospheric Administration (NOAA).
Efforts like Seabed 2030 aim to map the entire ocean floor by 2030, but progress is slow. The project relies on crowdsourced data from ships, but many areas are too remote for regular surveys. Until then, the true depth of the ocean will remain a mystery, with new trenches and seamounts still being discovered.
