The hydromagnetic structure of a neutron star accreting symmetrically at both magnetic poles is calculated numerically as a function of accreted mass, starting from a centered magnetic dipole and evolving through a quasistatic sequence of two-dimensional Grad-Shafranov equilibria. Our calculation is the first to solve self-consistently (by respecting flux freezing) for the distribution of accreted matter and magnetic flux as a function of stellar latitude. We find that 10^-5 M_solar must be accreted before the magnetic field is distorted enough to appreciably reduce the magnetic dipole moment, well above earlier estimates of 10^-10 M_solar. Moreover, the magnetic field is compressed locally up to 10^3 times its initial strength, deforming the crust significantly and creating a sizable, inclined mass quadrupole (epsilon ~ 10^-7 for 10^-1 M_solar accreted) --- a polar magnetic mountain.

We predict the amplitude of the gravitational wave signal from objects undergoing magnetic field burial by accretion, principally X-ray millisecond pulsars, and calculate their evolution in the spin period-magnetic moment plane. We also investigate related processes that affect the evolution of the mass quadrupole and hence the gravitational wave signal: detachment of magnetic bubbles above 10^-4 M_solar, disruption of the distorted magnetic field by hydromagnetic (Parker and interchange) instabilities, sinking of the magnetic mountain, and relaxation by Ohmic diffusion.