Malona

Now the asteroid belt that lies between Mars and Jupiter, it was once a home for over 400,000,000 human beings who were descendants from the genetically manipulated peoples that originated from the Sirius star system, long ago.

Malona, also called Phaeton, is mentioned by Semjase in the Contact Report 004:

109. Man on Earth must get accustomed to the thought that his predecessors have forced all of mankind and Earth itself to the brink of ruin, and had to partially evacuate the planet in a wild escape.

110. It should also serve as a warning to him that thirst for power and barbarism are attributes that bring death.

111. A second race also had to experience this truth in your solar system.

112. In their unlimited hatred and unquenchable thirst for power, they destroyed themselves right down to the last man, and scarcely a creature survived the conflict.

113. They eliminated and destroyed their own planet with a huge explosion and nothing remained of it but the many thousands of asteroids, which today still circle your sun — as a reminder of the human beings’ deadly irrationality.

114. (These asteroids are) remnants of the once thriving planet "Malona", which was destroyed by its inhabitants in their barbarian thirst for power and irrationality.

Billy also mentioned how Malona was destroyed in Contact Report 251:

Malona, or rather Phaeton, was destroyed in fratricidal wars by its genetically-manipulated inhabitants who had originally come from the Sirius regions, and who blew up the planet by diverting parts of an ocean into the subterranean or underwater magma chambers of a gigantic volcano. The SOL System's Asteroid Belt contains fragments of this former planet, whose orbit was not where the Asteroid Belt is today but was located where Mars orbits the Sun today, and Mars at one time was located where the asteroid belt is today. This position reversal was caused by immense upheavals in which the Destroyer (planet) played an important role. The planetary locations of Mars, Earth and Malona/Phaeton, and solar systems on the other side of our galaxy, in the Milky Way, where the two yellow races settled down, were selected as effective hide-outs by these races' benefactors, who were the conspirators for the escape of the genetically-manipulated people.

Also mentioned in Contact Report 251 is how the inhabitants of Malona constructed cities, pyramids, stations and other things:

Upon leaving the Sirius regions, the other genetically-manipulated peoples found a way to the SOL System, where several thousands of rebellious genetically-manipulated people had previously been banished to a myriad of terrestrial locations as a punishment. These masses of diverse races found refuge on the planets Mars and Malona/Phaeton, respectively, where they constructed cities, pyramids, stations and other things.