The Aquarius Project presents the most comprehensive convergence study of a Milky Way-sized dark matter halo, simulating six ultrahighly resolved halos to estimate halo-to-halo scatter in substructure statistics. The largest simulation resolves nearly 300,000 gravitationally bound subhalos within the virialized region. Simulations with mass resolution up to 1800 times higher accurately reproduce subhalo properties, showing good convergence for substructure abundance and internal properties. Subhalos within subhalos show up to four generations, but substructure in subhalos is less than in the main halo when compared at equal mean overdensity. The overall substructure mass fraction is much lower in subhalos than in the main halo. Extrapolating the main halo's subhalo mass spectrum down to an Earth mass predicts a substructure mass fraction of less than 3% within 100 kpc and less than 0.1% within the Solar Circle. Subhalo inner density profiles follow an Einasto form and do not converge to a fixed asymptotic slope. The mean concentrations of isolated halos are accurately described by the Neto et al. fitting formula down to 1.5 km/s. Subhalos are more concentrated than field halos, with a typical density 2.6 times larger and increasing with distance from the halo center. The Aquarius Project uses high-resolution simulations to study Galaxy-sized CDM halos, focusing on inner regions where dark matter density contrasts exceed 10^6. The simulations use GADGET-3, a parallel code allowing unprecedented dynamic range and numerical accuracy. Six simulations at high resolution, each with 160-224 million particles, are analyzed. The results show a power-law subhalo mass spectrum with a slope of -1.9, indicating a substructure mass fraction of about 4.5% within 100 kpc. The mass fraction in resolved substructures varies around 11% within r_50, with an average of 11.2%. The results are consistent with previous studies of galaxy cluster halos and Galaxy-sized halos, but larger than the 5.3% reported for a Milky Way-sized halo. The subhalo abundance per unit halo mass shows little halo-to-halo scatter, with an rms halo-to-halo scatter of only 8%. The concentration of halos is characterized by the mean overdensity within r_max, with a simple measure based on the NFW density profile. The characteristic NFW overdensity is 7.213 times the mean overdensity within r_max. The Aquarius Project provides detailed insights into the structure and evolution of dark matter halos and their substructures, with implications for dark matter detection and the nature of dark matter.The Aquarius Project presents the most comprehensive convergence study of a Milky Way-sized dark matter halo, simulating six ultrahighly resolved halos to estimate halo-to-halo scatter in substructure statistics. The largest simulation resolves nearly 300,000 gravitationally bound subhalos within the virialized region. Simulations with mass resolution up to 1800 times higher accurately reproduce subhalo properties, showing good convergence for substructure abundance and internal properties. Subhalos within subhalos show up to four generations, but substructure in subhalos is less than in the main halo when compared at equal mean overdensity. The overall substructure mass fraction is much lower in subhalos than in the main halo. Extrapolating the main halo's subhalo mass spectrum down to an Earth mass predicts a substructure mass fraction of less than 3% within 100 kpc and less than 0.1% within the Solar Circle. Subhalo inner density profiles follow an Einasto form and do not converge to a fixed asymptotic slope. The mean concentrations of isolated halos are accurately described by the Neto et al. fitting formula down to 1.5 km/s. Subhalos are more concentrated than field halos, with a typical density 2.6 times larger and increasing with distance from the halo center. The Aquarius Project uses high-resolution simulations to study Galaxy-sized CDM halos, focusing on inner regions where dark matter density contrasts exceed 10^6. The simulations use GADGET-3, a parallel code allowing unprecedented dynamic range and numerical accuracy. Six simulations at high resolution, each with 160-224 million particles, are analyzed. The results show a power-law subhalo mass spectrum with a slope of -1.9, indicating a substructure mass fraction of about 4.5% within 100 kpc. The mass fraction in resolved substructures varies around 11% within r_50, with an average of 11.2%. The results are consistent with previous studies of galaxy cluster halos and Galaxy-sized halos, but larger than the 5.3% reported for a Milky Way-sized halo. The subhalo abundance per unit halo mass shows little halo-to-halo scatter, with an rms halo-to-halo scatter of only 8%. The concentration of halos is characterized by the mean overdensity within r_max, with a simple measure based on the NFW density profile. The characteristic NFW overdensity is 7.213 times the mean overdensity within r_max. The Aquarius Project provides detailed insights into the structure and evolution of dark matter halos and their substructures, with implications for dark matter detection and the nature of dark matter.