What is the "great black substance" that the Lilliputians discover on the ground?

The "great black substance" that the Lilliputians discover on the ground is actually Gulliver's hat. When Gulliver first arrives in Lilliput, he is so large compared to the tiny inhabitants that common objects, such as his clothing, become significant discoveries for them. The Lilliputians initially misinterpret his belongings, and Gulliver's hat is an example of how they consider anything associated with him to be a source of intrigue and confusion. His hat, as a large and unusual object to them, stands out against the ground, prompting their reaction to it. This misunderstanding reflects a central theme in "Gulliver's Travels," where perspective and size play crucial roles in the interactions between different cultures.