Spongebob 

  "The existence of Spongebob and his strange friends is the result of radiation from nuclear arms testing that was performed on the Bikini Atoll in the late '40s and early '50s. Since they live under the atoll, the town is known as 'Bikini Bottom'." Out of all these theories, this one makes the most sense. After all, the show was created by a former marine biologist. 

- Quotations from Reddit, got the info from Flavorwire.