Final answer:
The mythical place that Plato wrote about that had sunk into the ocean is Atlantis.
Step-by-step explanation:
The name of the mythical place that Plato wrote about that had sunk into the ocean is Atlantis. Plato described Atlantis as a powerful and advanced civilization that existed around 9,000 years before his own time. According to the story, Atlantis was a naval power that sought to conquer other civilizations, but eventually, it met its downfall and sank into the ocean.
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