The Xorda attack on Earth was an event that took place some time during Earth's 21st century CE, led by the alien race known as the Xorda. The aliens' attack was a result of Dr. Ivan Kintobor's decision to dissect a Xorda emissary and use the knowledge to create the robot Isaac (as well as developing other new technologies).
Nearly 12,000 years ago, during Earth's 21st century, the Xorda came to the planet to offer an alliance to the human population. However, a science team led by Dr. Ivan Kintobor instead captured the Xorda's emissary and imprisoned it. After probing it for knowledge for various information, the team then dissected the alien. In response, the Xorda would wipe out all life on the planet. (StH: #124)
As vengeance for murdering their emissary, the Xorda bombarded the planet with Gene Bombs, which were made to genetically destroy only the human population but preserve the rest of the life on the planet as it was at the time. Many of Earth's human life was wiped out (in its current form), with the exception of a passenger craft which was in low Earth orbit during the attack. Their ship crashed into a mountain, and its surviving population went on to form the hidden city of Station Square. (StH: #106, #124)
Instead of preserving the planet's environment, the Gene Bombs caused massive volcanic eruptions which sent billions of tons of ash and dirt into the planet's atmosphere. This is known as the First Day of Fury in Echidna theology. It also resulted in the creation of the Chaos Emeralds, and the rapid evolution of the animal kingdom which formed Mobians (and possibly the Mutates) as well as creating a new breed of four fingered humans known as Overlanders. The Xorda attack on the planet marked the end of Earth and the beginning of Mobius. (StH: #148)
The original date of the Xorda attack in issue #124 was set at just over 3,000 years ago, which explained where the Mobian year began. The date was later retconned in issue #148 to have occurred around 12,000 years ago.