  We present a new pipeline for the efficient generation of synthetic
observations of the extragalactic microwave sky, tailored to large ground-based
CMB experiments such as the Simons Observatory, Advanced ACTPol, SPT-3G, and
CMB-S4. Such simulated observations are a key technical challenge in cosmology
because of the dynamic range and accuracy required. The first part of the
pipeline generates a random cosmological realization in the form of a dark
matter halo catalog and matter displacement field, as seen from a given
position. The halo catalog and displacement field are modeled with ellipsoidal
collapse dynamics and Lagrangian perturbation theory, respectively. In the
second part, the cosmological realization is converted into a set of intensity
maps over the range 10 - 10^3 GHz using models based on existing observations
and hydrodynamical simulations. These maps include infrared emission from dusty
star forming galaxies (CIB), Comptonization of CMB photons by hot gas in groups
and clusters through the thermal Sunyaev-Zel'dovich effect (tSZ), Doppler
boosting by Thomson scattering of the CMB by bulk flows through the kinetic
Sunyaev-Zel'dovich effect (kSZ), and weak gravitational lensing of primary CMB
anisotropies by the large-scale distribution of matter in the universe. After
describing the pipeline and its implementation, we present the Websky maps,
created from a realization of the cosmic web on our past light cone in the
redshift interval 0<z<4.6 over the full-sky and a volume of ~(600 Gpc/h)^3
resolved with ~10^12 resolution elements. The Websky maps and halo catalog are
publicly available at mocks.cita.utoronto.ca/websky.
