The stacking achievement requires two or more geohashers to occupy the hashpoint at different altitudes at the same time. The level of achievement gained depends on how many geohashers were stacked at different altitudes.
- A bridge over land would allow a level 2 stacking achievement, as would a tunnel through a traversable hill.
- A hashpoint in a multi-story carpark or a building staircase would afford a multi-level stacking.
- An apartment or office building with multiple ambassador achievements would also allow multi-level stacking.
- A simultaneous air and land geohash would be the ultimate but hard to prove!
The stacking must be accomplished by means of pre-existing terrain or structures, so these do not count:
- Stacking prone geohashers at the same altitude (e.g. during a Sex Geohash or while playing the Lying-Down Game).
- Stacking acrobatic geohashers at the same altitude (e.g. in a human pyramid during a Circus Geohash).
- Bringing a ladder is cheating.
The level gained depends on the number of distinct altitudes occupied simultaneously. For example, a bridge with two geohashers below and two geohashers above would only earn a level 2 achievement, not level 4.
 Positioning of the Geohashers
There must be a minimum vertical spacing of two metres between geohashers.
The usual allowance for GPS inaccuracy does not apply if it can be proved from the map that the stacking was impossible. For example, if geohasher A stands at the bottom of a sheer cliff, and geohasher B stands at the top, the achievement can easily be disproved by use of a map, even if they are only 1m apart in a bird's-eye view.
Proof for this achievement would include:
- A photograph showing how the stacking was possible (e.g. a photo of the outside of the carpark, or of the bridge showing the path underneath).
- A photograph of each geohasher at each level, and/or
- A photograph of each geohasher's GPS. As altitude measurements on GPS devices aren't very accurate, an alternative could be the GPS photographs showing the exact same time, and
- plus, a convincing story if the GPS cannot provide proof (e.g. one geohasher was underground, or one geohasher stood on a road bridge while another drove underneath)