Earth's gravity is so uneven that sea level isn't actually level. In the Indian Ocean, a massive "gravity hole" causes the water to dip by over 100 meters.
The standard gravitational acceleration of 9.8 meters per second squared is just an average. Earth's actual pull is patchy, fluctuating based on three main factors: the planet's shape, its rotation, and the uneven distribution of mass inside it.
Earth is not a perfect sphere. Because it spins, it bulges at the equator and flattens at the poles, making it an oblate spheroid. If you stand on the equator, you are about 21 kilometers (13 miles) further from the center of the Earth than if you stand at one of the poles. Since gravity weakens with distance, the pull is slightly weaker at the equator.
The spinning of the Earth also generates centrifugal force, which pushes outward. This force is strongest at the equator and zero at the poles, counteracting gravity just enough to shave a tiny fraction off your weight. If you climb a tall mountain, you lose even more weight by moving further from the planet's center of mass.
But the most complex variations come from what lies beneath the surface. Earth is not uniform on the inside. Differences in the density of rocks, magma chambers, and deep ocean trenches all alter the local gravitational field. Areas with dense subterranean rock formations, like the Andes Mountains, exhibit a slightly stronger gravitational tug.
To map these invisible fluctuations, scientists use satellites like NASA's GRACE mission, which track microscopic changes in the distance between twin spacecraft as they fly over denser or less dense regions of the planet. These measurements produce a "geoid"—a bumpy, potato-like map showing what global sea level would look like if it were shaped only by gravity, ignoring winds and tides.
If you want to experience the weakest gravity on Earth's surface, you need to go somewhere near the equator, high in elevation, and above low-density rock. The peak of Mount Huascarán in Peru currently holds the record for the lowest gravitational acceleration on the planet.