In the game Portal the player uses the wormhole placing abilities of the Aperture Science Handheld Portal Device to find her way through various obstacle courses. Aperture Science has not given the ASHPD to any geohasher. Thats why geohashers need to stick to non-interdimensional tunnels.
In this achievement, traversing tunnels (meaning everything tube-like that is longer than it's diameter) is encouraged. Try to cross as many subway stations, underpasses, children slides and so on. Every single tunnel (no matter of direction or how many times you transit) counts only once.
Take a photo of every tunnel you cross.
Show your success with a ribbon.
Everyone displaying the ribbon is listed in Category:Portal achievement.