While we are adept at three-dimensional geometry we are biased to see life constrained to 2 dimensions of the surface of the earth.  For the most part our position is sufficiently defined by latitude and longitude.  The altitude is where the terrain is at that point.   The existence of flying and diving creatures does not contradict this view because they need the surface for their survival or they need to stay relatively close to the surface.

Initially, we made a slight exception for the ocean where we recognize two surfaces: one for the sea level and one for the floor.  It is still two dimensional but just twice the area.  A lot of ocean life lives close to either of these surfaces.   Sea level has access to sunlight.  Ocean floor has access to shelter, minerals, and anchoring points.  These worlds do have some depth so it is a thin 3-dimensional block where the lateral dimensions still dominate.

The world most alien from our perspective is the middle of the ocean, especially in the deeper parts of the ocean.  In the column of water from the surface to the floor there are three dimensional habitats that are stacked on top of each other.  The habitats may have some vertical boundaries based on sunlight, salinity, or pressure, but the habitats offer a huge range of depths that can accommodate organisms.  Species can be comfortable lingering at any particular depth in this volume, and may productively obtain nutrients equally effectively by moving vertically or laterally.

For this middle depth habitats, there is little to suggest an absolute coordinate system.  Everything is relative.  There is no special place.

This is certainly over-simplified because the middle depth habitats are complex.  There are areas of different currents that may be sought or avoided.  Different depths offer different advantages for feeding and shelter.

But I’m over simplifying for to consider a world view very different from the 2-dimensional surface based one we take for granted.  What is the world-view of advanced intelligence that evolved to take for granted a 3 dimensional world where the three dimensions are roughly equivalent for the sake of living.  When seeking whether there is another species that is comparable in intelligence to itself, it is going to think in three dimensions.  This is in contrast to our view that there must be a surface.

The middle depth habitats are fascinating.  It is fascinating that they can exist at all, that there can be life that is not anchored to a surface for nutrients or shelter.  Even more fascinating is that there are multiple such habitats stacked on top of each other in the deepest trenches.    For these habitats, the base of the food chain is a combination of debris and life moving vertically, either drifting down from above, or pushed up from below by currents.  But each habitat is occupied by different mix of organisms.

There does not seem to be a limit to how deep life can live.  Each time we get deeper we find more life.   Even the floor of the deepest trench hosts abundant life.   Digging into that surface, we find more life.

Life exists vertically.  Pretend that a jelly fish has human-like intelligence.  For a living it navigates vertically to take advantage of multiple vertical habitats for food or shelter.  It may accept there is a depth below which it is unable to descend (perhaps due to pressure), or a height above which it can not rise (the surface).  But it would not have a problem speculating that there may be life above and below these limits.

In contrast, we are surprised to find life embedded in cores retrieved from rock thousands of feet below the surface.  We are surprised to find evidence of life in the stratosphere that improbably could have come from the lower troposphere.    Deep sea intelligence may not be surprised at all.

So why stop at the stratosphere?

When we look outwards for signs of extra-terrestrial life, we assume it will live on a world with a 2 dimensional surface.  We are looking for planets.   Outside of the solar system, we are looking for stars that may host planets.   This is our bias that life will be on a surface of a substantial body we would call a planet that is mostly likely to be found near stars.

There may be such surfaces.  But it may be possible that most of the terrestrial biology exists in space between the stars or at the far fringes of stars.  Perhaps the norm is for life to anchor itself to some icy or rocky body that is the size of a rock or dust particle.  The life may find a way to drift in space to find a new body to call a home.  This life may find it wise to develop mechanisms of camouflage to protect its home from predation.  It may find it advantageous to never be detected. 

It may prefer the habitats at the far edge of stellar systems such as the Oort Cloud.  Perhaps most star systems have similar clouds with similar levels of hospitality.   Life can spread in the occasion when clouds overlap during close encounters with another star system.  For this life, the less hospitable inner stellar systems would be avoided because there is no need to go there.  Life is good where they are at. 

An intelligence accustomed to a three-dimensional biosphere probably assume this as more likely.