Figure 1: (a) Extremal orbits on a warped Fermi cylinder for two different magnetic field orientations. (b) Quantum oscillations of the skin depth in underdoped $\text{YBCO}$ as a function of the magnetic field strength (Note: Oscillations are periodic only when plotted against $1/{\mu }_{0}H$) and (c) their Fourier transform. The latter reveals three closely spaced frequencies (in kilotesla) arising from orbits on small Fermi cylinders as in (a).