To illustrate the dangers of deepfakes, a team from the MIT Center for Advanced Virtuality created a deepfake of President Nixon delivering the real contingency speech written in 1969 for a scenario in which the Apollo 11 crew was unable to return from the moon. The video is called In Event of Moon Disaster and features both deepfaked video and audio. According to MIT News:
The team worked with a voice actor and a company called Respeecher to produce the synthetic speech using deep learning techniques. They also worked with the company Canny AI to use video dialogue replacement techniques to study and replicate the movement of Nixon's mouth and lips. Through these sophisticated AI and machine learning technologies, the seven-minute film shows how thoroughly convincing deepfakes can be.
You can watch the entire In Event of Moon Disaster video here. The Nixon part starts at about 3:45. It's fairly convincing, though there are some head nods that seem contextually out of place and the shirt collar feels a bit off. It's not something somebody would notice if they didn't already know the video was fake, and honestly it might just be in my head because I'm trying to find flaws. You could play me a real video of a Nixon speech and if you told me it was deepfaked I'd explain in great detail that I can tell because of the pixels. They're off.
Scientific America has a 30 minute documentary on how the Nixon deepfake was made which you can watch here.