John P. Harrington is known for taking copious notes on Southern California native languages. In the early twentieth century, many of these languages were already moribund. In at least one case - Tataviam, a Uto-Aztecan language spoken in the Santa Clara River basin which empties into the Pacific between Oxnard and Ventura - Harrington's notes constitute the only attestation the now-extinct language left to posterity.
It always amazes me when languages go extinct, not in isolation in the wilderness, but in the midst of booming humanity. Yes, I understand that the more numerous and economically-overbearing the new language community is (English in this case), the more likely the old language is to go extinct. But it always seems that out of all these people that settled Southern California, more than one would have been interested in recording them before they disappeared into the dust.