Hollow Moon

No scientific evidence exists to support the idea; seismic observations and other data collected since spacecraft began to orbit or land on the Moon indicate that it has a solid, differentiated interior, with a thin crust, extensive mantle, and a dense core which is significantly smaller (in relative terms) than Earth's.

[2] This idea dates from 1970, when two Soviet authors published a short piece in the popular press speculating that the Moon might be "the creation of alien intelligence"; since then, it has occasionally been endorsed by conspiracy theorists like Jim Marrs and David Icke.

[9] Mainstream science argues this difference is due to the fact that the Earth's upper mantle and crust are less dense than its heavy, iron core.

[11] In 1970, Michael Vasin and Alexander Shcherbakov, of the Soviet Academy of Sciences, advanced a hypothesis that the Moon is a spaceship created by unknown beings.

[14] The authors reference earlier speculation by astrophysicist Iosif Shklovsky, who suggested that the Martian moon Phobos was an artificial satellite and hollow; this has since been shown not to be the case.

[15] Skeptical author Jason Colavito points out that all of their evidence is circumstantial, and that, in the 1960s, the atheistic Soviet Union promoted the ancient astronaut concept in an attempt to undermine the West's faith in religion.

[2] In 1965, author Isaac Asimov observed: "What makes a total eclipse so remarkable is the sheer astronomical accident that the Moon fits so snugly over the Sun.

The Moon is just large enough to cover the Sun completely (at times) so that a temporary night falls and the stars spring out.

They note that the angular diameters of Sun and Moon vary by several percent over time and do not actually "perfectly" match during eclipses.

Seismic observations have been made, constraining the thickness of the Moon's crust, mantle and core, demonstrating it could not be hollow.

[40] These reflectors made it possible to measure the distance between the surfaces of the Earth and the Moon using extremely precise laser ranging.

True (physical) libration of the Moon measured via Lunar laser ranging constrains the moment of inertia factor to 0.394 ± 0.002.

[45] This was accomplished by measuring the Doppler shift in the S-band tracking signal as it reaches Earth, which can be converted to spacecraft accelerations.