In this article we will retrace one of the great mathematical adventures of this century-the discovery of the soliton and the gradual explanation of its remarkable properties in terms of hidden symmetries. We will take an historical approach, starting with a famous numerical experiment carried out by Fermi, Pasta, and Ulam on one of the first electronic computers, and with Zabusky and Kruskal's insightful explanation of the surprising results of that experiment (and of a follow-up experiment of

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... their own) in terms of a new concept they called "solitons". Solitons however raised even more questions than they answered. In particular, the evolution equations that govern solitons were found to be Hamiltonian and have infinitely many conserved quantities, pointing to the existence of many non-obvious symmetries. We will cover next the elegant approach to solitons in terms of the Inverse Scattering Transform and Lax Pairs, and finally explain how those ideas led step-by-step to the discovery that Loop Groups, acting by "Dressing Transformations", give a conceptually satisfying explanation of the secret soliton symmetries.