Simple Celtic key pattern inside an overall Latin Cross shape.  This is partially loosely based on a design in "Celtic Art: The Methods of Construction" by George Bain, which was in turn loosely based on patterns in the Book of Kells.

If the lighter green path were to be cut somewhere on the outside of the cross, it would become a labyrinth which returns to its starting point.
Self-made image, generated from the following vector PostScript source code:
%!
306 484 translate 2.4 dup scale/A 1800 sqrt 1 add def/B 1800 sqrt 5 sub
dup mul dup add sqrt def/C B 5 sub def/F B 15 sub def/D F 2 div def/E B
2 sqrt div 2 mul def 0 .1667 0 setrgbcolor A neg A moveto
A neg A E add lineto A A E add lineto A A lineto A E add A lineto
A E add A neg lineto A A neg lineto A A neg E 2 mul sub lineto
A neg A neg E 2 mul sub lineto A neg A neg lineto
A neg E sub A neg lineto A neg E sub A lineto closepath fill
8 setlinewidth 0 .6667 0.1111 setrgbcolor gsave 0 E translate 45 rotate
5 C neg moveto C -5 lineto 15 -5 lineto 15 D neg lineto 5 D neg lineto
5 D lineto 15 D lineto 15 5 lineto C 5 lineto 0 B lineto C neg 5 lineto
-15 5 lineto -15 D lineto -5 D lineto -5 F neg lineto -15 F neg lineto
-15 -5 lineto B neg -5 lineto stroke grestore gsave E neg 0 translate
45 rotate B -5 moveto 15 -5 lineto 15 F 10 sub neg lineto
F 10 sub F 10 sub neg lineto F 10 sub F neg lineto 5 F neg lineto
5 D lineto 15 D lineto 15 5 lineto C 5 lineto 0 B lineto C neg 5 lineto
-15 5 lineto -15 D lineto -5 D lineto -5 D neg lineto -15 D neg lineto
-15 -5 lineto C neg -5 lineto -5 C neg lineto stroke grestore gsave
45 rotate B 5 add neg 5 moveto -15 5 lineto -15 F lineto -5 F lineto
-5 F neg lineto F 10 sub neg F neg lineto
F 10 sub neg F 10 sub neg lineto -15 F 10 sub neg lineto -15 -5 lineto
B 5 add neg -5 lineto stroke B 5 add -5 moveto 15 -5 lineto
15 F neg lineto 5 F neg lineto 5 F lineto F 10 sub F lineto
F 10 sub F 10 sub lineto 15 F 10 sub lineto 15 5 lineto
B 5 add 5 lineto stroke grestore gsave E 0 translate 45 rotate
B neg 5 moveto -15 5 lineto -15 F 10 sub lineto
F 10 sub neg F 10 sub lineto F 10 sub neg F lineto -5 F lineto
-5 D neg lineto -15 D neg lineto -15 -5 lineto C neg -5 lineto
0 B neg lineto C -5 lineto 15 -5 lineto 15 D neg lineto 5 D neg lineto
5 D lineto 15 D lineto 15 5 lineto C 5 lineto 5 C lineto stroke
grestore gsave 0 E neg translate 45 rotate -5 C moveto C neg 5 lineto
-15 5 lineto -15 D lineto -5 D lineto -5 F neg lineto
F 10 sub neg F neg lineto F 10 sub neg F 10 sub neg lineto
-15 F 10 sub neg lineto -15 -5 lineto B 5 add neg -5 lineto stroke
5 C neg moveto C -5 lineto 15 -5 lineto 15 D neg lineto 5 D neg lineto
5 F lineto 15 F lineto 15 5 lineto B 5 lineto stroke grestore gsave
0 E 2 mul neg translate 45 rotate -5 C moveto C neg 5 lineto
-15 5 lineto -15 D lineto -5 D lineto -5 D neg lineto -15 D neg lineto
-15 -5 lineto C neg -5 lineto 0 B neg lineto C -5 lineto 15 -5 lineto
15 D neg lineto 5 D neg lineto 5 F lineto 15 F lineto 15 5 lineto
B 5 add 5 lineto stroke grestore 2 setlinewidth 0 .1667 0 setrgbcolor
A 1 sub neg A 1 sub moveto 5 -5 rlineto stroke
A 1 sub A 1 sub neg moveto -5 5 rlineto stroke showpage
%EOF
The following is the vector PostScript source code for the basic square, six of which are used to form the cross:

%!
300 396 translate 4 dup scale/A 1800 sqrt 1 add def/B 1800 sqrt 5 sub
dup mul dup add sqrt def/C B 5 sub def/D B 15 sub 2 div def A A moveto
A neg A lineto A neg A neg lineto A A neg lineto closepath fill gsave
45 rotate 8 setlinewidth 1 setgray -15 5 moveto -15 D lineto -5 D
lineto -5 D neg lineto -15 D neg lineto -15 -5 lineto C neg -5 lineto 0
B neg lineto C -5 lineto 15 -5 lineto 15 D neg lineto 5 D neg lineto 5
D lineto 15 D lineto 15 5 lineto C 5 lineto 0 B lineto C neg 5 lineto
closepath gsave stroke grestore showpage
%EOF
AnonMoos