This is a map of something invisible – ocean currents – made indirectly – by releasing messages in bottles and seeing where they land, based on who sent back the enclosed slip from where.
Nowadays, indirect observations like this are a little more complicated, but the same basic principles are still at play in things like astronomy, space travel, meteorology, and medicine. If you can’t see it, try to find some way to watch what it does.
[via nemfrog]