Gravity
The best scientific argument against that of a hollow Earth (or in fact any hollow planet) is gravity. Massive objects tend to clump together gravitationally, creating non-hollow spherical objects we call stars and planets. The solid sphere is the best way in which to minimize the gravitational potential energy of a physical object; having hollowness is therefore unfavorable in the energetic sense. In addition, ordinary matter is not strong enough to support a hollow shape of planetary size against the force of gravity.
Someone on the inside of a hollow Earth would not experience an outward pull and could not stand on the inner surface; rather, the theory of gravity implies that a person on the inside would be nearly weightless. This was first shown by Newton, whose shell theorem mathematically predicts a gravitational force (from the shell) of zero everywhere inside a spherically symmetric hollow shell of matter, regardless of the shell’s thickness.
A tiny gravitational force would arise from the fact that the Earth does not have a perfectly symmetrical spherical shape, as well as forces from other bodies such as the Moon. The centrifugal force from the Earth’s rotation would pull a person (on the inner surface) outwards if the person was traveling at the same velocity as the Earth’s interior and was in contact with the ground on the interior, but even at the equator this is only 1/300 of ordinary Earth gravity.
The mass of the planet also indicates that the hollow Earth hypothesis is unfeasible. Should the Earth be largely hollow, its mass would be much lower and thus its gravity on the outer surface would be much lower than it currently is.Seismic Information
Although not visually observable, the core of the Earth is observable via vibrations (primarily from earthquakes) passing from one side of the planet to the other. Using this method, geologists have been able to establish the structure of mantle, outer core, and inner core known today. A hollow earth would behave entirely differently in terms of seismic observations.Visual Evidence
The deepest hole drilled to date is the SG-3 borehole which is 12.3 km (7.6 miles) deep, part of the Soviet Kola Superdeep Borehole project; thus, visual knowledge of the Earth’s structure extends that far.
