The Hidden City Initiative is a project in the Sonic the Hedgehog comic book series and its spin-off comics. It was a project carried out by humanity during the 21st century after the declaration of war by the Xorda.
In the early 21st century, the Xorda ambassador was captured and dissected when it first arrived on Earth. In retaliation, the aggressive Xorda race declared war on Earth. Fearing the impending full-sale Xorda invasion and destruction of humanity, the Hidden City Initiative was enacted. For this project, a handful of small, self-sufficient cities were constructed in concealment and out of view from the Xorda invasion, which were meant to preserve humanity. At the time the Xorda launched their Gene Bombs, only a fraction of the human population had been evacuated to these cities.
Over the next thousands of years, as the Earth's surface changed radically over the eons and gave birth to new forms of life, the hidden cities created by the Hidden City Initiative established a network of communication and trade. In the present day, two of the cities created by the Hidden City Initiative, Soleanna and Station Square, still remains on Earth, now having been renamed Mobius, and have also included Overlanders into the population following the Second Robotnik War.