11. Their Manner of Communication Is A Mystery
We assume that the inhabitants of the North Sentinel Island speak Sentinelese but the truth is that we do not actually know what they speak or the words they use. The very first time that someone from the modern world heard sounds in a language that was believed to be Sentinelese was as late as 1980.
The researchers who were unable to communicate with the islanders other than by use of sign language understood that some basic form of spoken language was being used by the islanders to communicate within their group.