One of the most fascinating experiments was done by Bastian in 1960s (our first podcast will be about it once the site goes live). It remains highly controversial but has also been replicated by others. In 2016 one Russian scientist claimed to replicate it and it resulted in huge storm and controversy. Basically it involved 2 dolphins separated visually but not acoustically. One dolphin was shown what action the second dolphins should perform so both can get a fish. It appeared that dolphins did communicate and performed above the chance level.