system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 How do we know what percentage of marine life/insects/mammals/etc we've discovered if we dont know how many there could be overall?
gpt
Let's say we discovered 500 new species of large fish between 1800-1850, 400 between 1850 and 1900, 300 between 1900-1950, and so-on..

We can then model that we're unlikely to discover more than ~50 new species in the future, so the discovery rate is about 97%.

Of course, in real life, data is messier, but the same sort of general analysis can be made to estimate how many species we'll discover, based on how many we have discovered so far.