system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How are scientists able to claim what percentage of wildlife is undiscovered?

When scientists claim that 75% of wildlife in Australia is undiscovered, how do they come up with this? How are they able to know how much of the unknown is out there? I've seen figures, such as for ocean life, as well.
gpt
Go to a new area of Australia (or wherever). Comb one acre very carefully, cataloging every species. Figure out how many new species there are. Do this in a few different places, figure out about how many new species there are in general, and extrapolate.