These are puzzling structures, because they should not be here.
'Here' is literally in the middle of the Atlantic, the Azores islands.
Officially, the Azores were found uninhabited by
Portuguese navigators in the early 1400s.
However, physicist Dr Felix Rodrigues has been researching and dating
buildings believed to be much older than that.
Dr Rodrigues' quest to find answers started 10 years ago
in the Terceira Island, where he lives.
These are the oldest structures he's found so far.
Dr Rodrigues believes these are dolmens used by prehistoric cultures
to place remains of the dead.
These cylindrical cavities would be indicators for the human presence,
showing intentional work in the rocks.
Material accumulated inside the cavities has been dated to be
two and a half thousand years old, at least.
Ceramic pieces found in the area are believed to be even older,
over 4,000 years.
By the sea, there are some intriguing clues of this supposed arrival.
This other structure doesn't seem to have been built
by 15th Century Portuguese either.
For Dr Rodrigues, these two structures are part of the same ritualistic complex,
which is completed by a sort of ground basin in the middle of the Columbarium.
It seems to be connected to the outside by a duct.
Similar basins are found in another location in the Terceira island,
inside some perfectly carved volcanic rocks.
Next to it there is a flooded cave.
But not everyone is convinced this complex is that old.
Some say these could be stables from a 16th Century fortress.
In the inland, the origins of these strange tracks are also controversial.
Locally called 'relheiras', they are popularly associated with cart loads.
Some questions are easier to answer than others.
But we are still far from knowing for sure who this civilisation was.