What is the lowest altitude at which the curve of the earth can be seen?

The curvature of the Earth becomes noticeable at altitudes above approximately 35,000 feet (about 10,700 meters), which is roughly the cruising altitude of commercial airliners. At lower altitudes, the horizon appears relatively flat due to the Earth's vast size. However, some observers claim to see a slight curvature from altitudes as low as 10,000 feet, especially with a wide panoramic view.