This is just an exploration into the possible nature of the aliens, or extraterrestrials, that are 'visiting' Earth.

Without a doubt, the UFO phenomenon is very complex, and very elusive. The UFO occupants show their craft or themselves only for a short time, leaving barely any evidence of their presence in our world. Investigators have long pondered about who these beings are and where they come from. Originally it was thought they were physical extraterrestrials from another planet coming here to pay us a visit, but further investigation have brought to light that this can't explain many features of UFO or alien encounters. The aliens don't talk much, and when they do they are vague. So, we can only speculate based on an analysis of the encounters.

Although the most known types of aliens are the Grays because of the major interest in abduction cases, there are many other types of aliens encountered. They all seem to be humanoid.

Physical ETs

Although I can't exclude it, I find it unlikely that a physical ET race, in the sense that they would be similar to humans, would be able to land here on Earth, leave their craft and walk around. Life forms on a planet are always adapted to their specific environment. Each planet has a different atmosphere, composition of its gases and atmospheric pressure; and a difference in gravitation, solar and cosmic radiation, light intensity and microbial life. And yet, we see the ETs leave their ship and walk around as if nothing is the matter. Are they not afraid that they would load themselves up with potentially devastating viruses, bacteria and fungi against which they haven't an immune defense? Apparently not. being technologically advanced they seem to have solved at least that problem. In one account, the abductee said that "...there was some kind of machine that, I was told by mental telepathy, could analyze if there was anything wrong in my body and could also kill bacteria that wasn’t good..." So we might assume that if physical ETs can come all the way here from some place in the universe, that they have at least advanced technology that can decontaminate their bodies and their craft.

Technology

If we want to understand ETS, we also have to look at their technology. Most of their technology, if not all of it, has to do with high frequencies, high-frequency-pulses, and intense energies. The only way you can travel the vast distances in the universe is to exceed the speed of light. To do that the ship's mass (which is just a property of what we call matter), and everything in it, needs to be raised until it becomes mass-less from our physical perspective. The ship is then in an energy state beyond our physical dimension, and beyond the light speed barrier. The higher dimension is non-local space, where you can travel from one place to another in very little time.

When these spaceships appear here on Earth, they are lowering their energy state, thereby gaining mass again. That is why UFOs are most often seen in their high energetic state, that is, as light spheres or plasma balls. Even when they are physical again, their craft can still be surrounded by plasma as the field around the craft is intense enough to turn the surrounding air into a plasma state. The high frequency, intense energy state of the craft can result in a lot of harmful effects on soil, plants, animals and humans nearby. Read my article of The Dangers of UFOs and Alien Encounters.

So, if physical ETs come here they have to travel in the non-local space.

Energy

Everything about ETs is about these high frequency, intense energies. Their technology is so evolved that they can manipulate energy and matter is all its aspects. Different ET races have different technologies, so they can't always do the same thing. We see that in the healings they perform. Some of them use physical equipment, perform surgeries, use ointments. Some use devices that can scan the interior of the body, even map the meridians. Lights are frequently used, and light beams. Most of the healings actually happen with light, at least that is what is perceived by the people involved. It is not ordinary light as we know it.

We, humans are primarily focused on the material world, but ETs are all familiar with the higher dimensions, and also the subtle energies of the human body. They know how to manipulate these subtle energies and even to instantly transform them. There are healings in which an ailment, disease or injury was instantly (in a matter of seconds or minutes) cured by a light beam (inside a craft), even from a distance when a light beam was emitted from a UFO hitting the witness. It is beyond out comprehension, but it shows that they have knowledge of matter/energy in all its states, and how to manipulate it.

A higher physical frequency

I think that only a small percentage of the visiting ETs are physical beings as most people understand it. Earth is situated at the outer edge of the Milky Way Galaxy where physical matter is of a slow vibration. The closer we go to the center of the galaxy, the higher the vibration of matter. Beings of those regions will be of a higher physical frequency, or shall we say a substance frequency. With some alien contacts, we see that the ETs don't adhere to our physical laws. This might be due to their advanced technology, but we also see them do other things, just as floating down a light beam, floating over the surface of the ground, passing through walls etc. In some instances of healing, the hand of the alien goes right into the body of the abductee.

Dimensions

So, we need to understand that the physicality of other beings in the universe is not always the same as the physicality of our world. There are also other dimensions, in the esoteric teaching called the astral realms, which are of a much higher vibration. ETs and beings that belong to Earth's higher realms, can lower their frequency and show up in our physical world. These type of beings can appear as light beings, and some ETs are seen as being composed of light, or emitting or enveloped in a light halo.

Some aliens appear first as light sphere and then morph into a humanoid appearance. Or they step out of a light sphere; in this case there might also be technology involved.

We, humans live a physical life, and after death, we fashion ourselves an astral body to continue living in the astral realm until we incarnate again. Our astral body takes the shape of the previous physical body because the mind is a creature of habit.

Some spiritually advanced ETs have physical lives as well, but when they are in spirit body they also will take the form of their previous physical body as well, out of habit. They can also fashion themselves a spaceship, because thought shapes astral matter in any form. Whether they live on the astral plane temporarily, or permanently, they can appear here in their astral spaceships and bodies. These beings can change the form and appearance of their ships and bodies. Usually they appear to us in a visible form that resembles a humanoid or very human-like appearance.

Many of these human-like ETs seem to be of these higher dimensional beings. When they perform healings, they do not need any technology. They can heal with their hands, sometimes directly with their mind without touching the person. Some of them do use some sort of astral technology based on light and frequencies.

Greys, Mantis and Reptilians

It seems that the so-called Greys are physical beings with a vast array of technology, with which they can do many things beyond our understanding. There are some indications that with their technology they can shift their physical frequency to the etheric or astral plane and back. This would allow them to walk through walls, move people from their bedroom through the walls too, and straight into a ship without passing through a door.

The Greys might also be of an etheric nature, considering their range of activities, that is, they exist between the astral and the physical world. One could say that their bodies are physical but of a slightly higher vibration than our physical world, but not high enough to be of an astral, fluidic nature. So they have substance but not of a gross-physical nature. Being of etheric nature, it is much easier for them to hop between the etheric realm and the physical world. Maybe that is why it is so difficult to figure out if the abductions they carry out are of a physical nature or by way of an out-of-the-body-experience. If the Greys are of an etheric nature, than their ships are too, but when they lower the vibration, the ships assume a quasi-physical state. They look physical, and can even be tangible, but they are not composed of physical materials. It's like a condensed form of a higher vibrational substance. Their bodies are then of the same nature.

If it is true that the Greys are involved in creating alien-human hybrids (in which they want to incarnate in order to be able to live on the surface of this planet) than we we are looking at physical Greys. If physical they are certainly be able to also work in the etheric and astral dimension, as they have also been seen active on these levels as evidenced by out-of-the-body experiences, hallucinogenic experiences and even in some rare cases of near-death experiences.

Possibly, there are different types of Grey races, as some contactees/abductees have expressed. This would also mean that their nature of being also differs. Some might be entirely physical, while others are etheric, or astral.

Little is known about the Mantis. They seem to be physical. On a rare occasion they are seen in the woods cloaking themselves upon discovery.

The Reptilians are most certain physical, and live underground, although in small numbers. However there are also reports that in their spirit state they are also actively interfering in our lives.

There is also a race of Blue ETs who say they once were physical beings living on another planet, but a section of their population have learned to transform themselves into light beings in order to solve an overpopulation problem. The blue light beings are a kind of transformed light-substance beings. You can find more about these beings in my article of Who is Behind Alien Abduction? I mention them here to show that there are the nature of intelligent beings in the universe is varied, and sometimes in ways we don't understand (yet). In close encounters it is not always easy, and sometimes impossible to figure out what the exact nature of the ET, or aliens, is.