Here is greater detail about how the Viking homes were hidden and photo
references.
It's very easy to see how a flat roof with dirt piled against the side
walls would make the reproductions of the Viking dwellings at L'anse
Aux Meadows (shown here
http://www.michael-thomann.com/nf/foto/bilder/antony1.jpg ) blend in
with the hillocks that dot the area (shown here
http://community.webshots.com/photo/162143049/1162239312012644958kqrZkw
) or be mistaken for a continuation of the row of hillocks in the
background of this photo
(http://community.webshots.com/user/weldonschloneger/2 ).
Here is a modern dwelling that is also at L'anse Aux Meadows which,
when seen from the left side, is probably hidden. (shown here
http://artandcarol.ca/artandcarol/GNP5.jpg . I don't know when I have
seen better photographs and greater skill with a camera than their
extensive artandcarol.ca website.) The Vikings also piled rocks up
against the walls (they also served as insulation) so it is easy to see
how it appeared to be another hillock from the sea and still did, at
least until you walked across the roof and saw the door on the inland
side of the longhouse and other buildings.