Ancient Gauls only feared one thing: That the sky might fall on their heads. They did not worry about Quantum Gravity, presumably because it was considered irrelevant at energies accessible at the time. This viewpoint, which has survived to our day, is challenged by the ideas of the Swampland Program, which posits that gravity has a subtle but universal imprint even at low energies. I will introduce the Swampland idea, the most widely studied Swampland constraints, and explain how these have been used to make predictions both for String Theory and phenomenology, including a micrometer-sized extra dimension.