Humans are not alone on Earth as intelligent species. Where only one intelligent species is developed, per planet, for 3rd Density existence, the Earth has some transplants. You are familiar with Bigfoot, who lives on the surface, and the Men in Black, transplants who live underground, who are hominoid groups and are in 3rd Density as are humans. Little known is that a 3rd Density reptilian race also lives underground, in the caverns and waterways that occur naturally. This reptilian race, if seen by humans, would not be recognized as an intelligent species, as they carry none of the trappings of civilization. Do they have technology, structures, computers? Yes and no. Where their IQ is less than humans and they are not as dexterous nor upright, they nonetheless can maneuver their surroundings. Socially, their 3rd Density existence is not that much different from humans. They both care for and murder each other. We will not mention their form, nor the locations of their homes, as they would be vulnerable should humans go hunting for them, for sport, as those inclined toward the Service-to-Self orientation surely would.
These transplants to the Earth came from worlds that could no longer support them in their 3rd Density existence. There, they lived a surface existence. Both subterranean transplants arrived after man had undergone considerable genetic engineering, but before he became what you would term civilized. They understood the situation, that they would have considerable time to acclimate themselves before mankind, with its violent nature, might discover them.
The Men in Black were a secondary intelligent species on their planet of origin, the primary race having undergone a Harvest or Transformation into 4th Density. As with most Transformations, the intelligent species transforming is evolved to support 4th Density existence and is no longer available for 3rd Density existence. After the Transformation, enough fauna and flora remained in 3rd Density on this planet to support the development of yet another intelligent species. As the mass of the sun in this solar system was partially migrated to 4th Density, it was much dimmer for this second development cycle. This is why the Men in Black cannot tolerate life on the surface. They were used to dim light. The Men in Black and humans are not connected in any way except that they both are hominoid. The Men in Black in fact are not even biologically compatible with humans to the degree that offspring might be possible, a situation that occurred with the hominoid visitors from the 12th Planet. The reptilian race developed on a water planet. They also suffered from a dying sun, and were used to cold, dark conditions before they were transplanted.
Each intelligent species is on its own time cycle, during 3rd Density, the Harvest occurring periodically at regular intervals. Were your Earth not scheduled to become a home for 4th Density existence, with a Transformation that will leave the Earth essentially a dead planet in 3rd Density, continued human existence in 3rd Density would be possible, for instance. In the case of the two subterranean transplants, the timing of their cycles does not coincide with Homo Sapiens' cycle. Close, but lagging somewhat behind. Both transplant species have had several Harvests, as have Homo Sapiens, and as with Homo Sapiens this Harvest is to be the last. Those incarnating entities who have not completed their 3rd Density spiritual lesson - determining their spiritual orientation - will all be incarnated elsewhere into yet another physical form. Transplanting will not occur.
Is there any truth to the stories of fabulous cities underground, and miles of tunnels bridging the continents? Very little. The stories of fabulous tunnel systems are a fraud, designed to blunt curiosity about the underground bunkers and tunneling that some governments, the United States included, have indulged in. Underground structures? Well, it might be this alien group, or that, but certainly not your government. If these fabulous tunnel systems are so real, why hasn't the media, or the public for that matter, been invited for a tour?