For instance, imagine taking one small patch of water sitting above the ocean floor a few miles out from the coast, and recording how many new species you find there. Perhaps you see a few crustaceans clinging to a seafloor boulder, several species of fish darting around, and a couple of sediment-feeders embedded in the silty seafloor. Then go back a second time and do it again, making a note of the number of species that you didn't see there before. Maybe this time a shark swims through your section of water, and you spot one or two other new creatures.
As you go on repeating this process, Horton says, you will tend find fewer and fewer new species. If you plot the number of new species you've found on a graph over time (and do "a load of statistical analysis called rarefaction", Horton adds), you will see a curve that starts out steep as you discover lots of new species, before flattening out towards the horizontal as it reaches what's called an asymptote – at this point, after many dives to inspect your patch of ocean, you have effectively described everything that lives there.
