I have to say I've never really found a maze that was challenging enough to take more than a minute or so to solve. Some of these mazes look much more difficult than anything I've seen before. Very cool and creative!
http://www.astrolog.org/labyrnth/maze/larger.gif
Enjoy!
Note: This is from the "Think Labyrinth!" site mentioned on the Maze Design site:
http://www.astrolog.org/labyrnth/maze.htm
It is also not the "largest maze" on that page by far; there are a couple of larger ones (as zip'd files), and likely those aren't the largest today.
I'm pretty certain they are just meant as "art" - and not to actually challenge someone to solve them; but if you are so inclined, go for it!
EDIT: I just wanted to add a bit more about the "dense maze" - the start/end is at the upper-left/lower-right of the maze; the white line is the "path" of the maze (black being the walls). Note how the image "changes" depending on the level of "zoom" - if you fit it to a smaller size, the "white" of the image will take a "cross shape", while other areas are "darker" - zoom in on those various areas, and notice how the path changes; it's a very complex construct!
Update final: After just over 4 hours it appears to have given up without actually filling anything.
Cue "I don't know what I expected" meme.