The explorer who crossed the isthmus of panama and sent back reports of a large sea on the other side was

The correct answer is Vasco Núñez de Balboa

This was during his third voyage and this meant that it was now possible to get to the Pacific ocean without circling all the way around South America which drastically reduced the amount of time it took to travel. Many people incorrectly believe that it was Magellan who did this but he went around the south.