Blue Whales 🐋 and some other baleen whale species call at such deep frequencies that they're undetectable by orcas, which cannot hear below 100 hertz.

Scientists grouped whale species into “fight” or “flight” sections depending on their response to orcas.

https://www.iflscience.com/blue-whales-one-of-the-loudest-things-on-earth-are-acoustically-invisible-to-killer-whales-77939