The ocean surface in one Indian Ocean spot dips 106 metres below average
Sea level is not flat. Earth bulges at the equator and gravity varies from place to place, so the ocean's resting surface sags as much as 106 metres below the global mean in one Indian Ocean hollow. Every country therefore picks its own zero, from a Cornish harbour to a Marseille tide gauge.
Mean sea level is an average height of coastal waters used as the baseline for measuring elevation. It serves as a chart datum for mariners and, in aviation, as the reference for calibrating altimeters and setting flight levels. Heights are quoted as above mean sea level, while above present sea level compares ancient shorelines with today's.
Pinning it down is hard because the sea never stops moving under tides, tsunamis, wind, air pressure, temperature, salinity and local gravity. Averaging out wind waves gives the still-water level; averaging that over long enough to cancel tides gives the mean. Official gauges may average hourly readings across a full 19-year Metonic lunar cycle. A gauge also sits on land that can itself rise or sink, so a change in relative sea level may reflect moving ground as much as moving water.
Earth's radius at sea level is about 6,378 km at the equator and 6,357 km at the poles. That squashed shape plus gravity anomalies defines the geoid, which approximates mean sea level in the open ocean. A study in 2026 reported that the geoid used in about 90 percent of published work put sea levels roughly 25 cm lower than measurements, understating sea level rise.
National zeros vary. Britain's maps start from the mean measured at Newlyn in Cornwall between 1915 and 1921, replacing an earlier Liverpool datum. Russia and several former imperial territories still measure from the Kronstadt gauge. Marseille's tide gauge has run continuously since 1883, the longest such record, and sets the reference for parts of Europe and much of Africa; the European system is calibrated to Amsterdam Peil from the 1690s. Since TOPEX/Poseidon launched in 1992, followed by the Jason satellites, altimeters in orbit have measured sea level precisely.
Source: Sea level
