The answer to this question can be found in Part II, Chapter 1 of Gulliver's Travels. After a return to England from the land of Lilliput, Gulliver returns to sea on a ship named the Adventure. Somewhere in the Indian Ocean, the ship was blown off course by a terrible storm, and was forced to anchor off the coast of a mysterious island to take on fresh water. Gulliver was part of a party sent ashore, and wandered off a bit before returning to see that the other men had already left on the boat, and were rowing back to the Adventure. He was, he says, about to "halloo after them" when he saw a "monster" chasing after them. It was a giant Brobdingnagian, and Gulliver flees the scene, into some fields, where he discovers that he is in a country of giants.