Multiple astronomical observations have established that about 85% of the matter in the universe is not made of normal atoms, but must be otherwise undetected elementary "dark matter" particles that do not emit or absorb light. Deciphering the nature of this so-called Dark Matter is of fundamental importance to cosmology, astrophysics, and high-energy particle physics. A leading hypothesis is that it is comprised of Weakly Interacting Massive Particles, or WIMPs, that were produced moments after the Big Bang. If WIMPs are the dark matter, then their presence in our galaxy may be detectable via scattering from atomic nuclei in detectors located deep underground to help reject backgrounds due to cosmic rays. Direct detection of WIMP dark matter would solve a fundamental mystery in particle physics and cosmology, providing a unique window to learning about the primary matter constituent of the Universe and of physics beyond the Standard Model of particle physics. This award is to support the NSF-funded DarkSide (DS) groups for the construction and operation of DarkSide-20k (DS-20k), a direct WIMP search using a Liquid Argon Time Projection Chamber (LAr TPC) with an active (fiducial) mass of 23 tons (20 tons).The LAr TPC relies heavily on the utilization of argon obtained from underground sources, or UAr. The planned Aria project for UAr purification may improve the worldwide availability of valuable stable rare isotopes such as O-18, N-15, and C-13, used for various medical, industrial and energy generation applications. The production of hundreds of tonnes of low-radioactivity UAr for DS-20k and for other technical uses requiring detection of Ar-37 for nuclear test ban verification, and Ar-39, for radiometric dating. Special E&O programs are planned with focus on educating K-12 teachers about basic physics and how it relates to dark matter detection, re-starting a summer school experience for high-school and undergraduate students, as well giving research opportunities to undergraduate students at participating underrepresented-minority serving institutions.DS-20k builds on the success of DarkSide-50 (DS-50), operating at LNGS since 2013, which produced two zero-background science results using atmospheric argon (AAr) and then underground argon (UAr) fills. DS-50 demonstrated an 39Ar reduction of 1400 in the UAr. Electron recoil (ER) background was shown to be completely suppressed in the region of interest, with discrimination better than 1 part in 1.5 x 10^7. These results demonstrated the ability of large LAr TPCs to operate in an "instrumental background-free mode" - a mode in which less than <0.1 events (other than actual nuclear recoils from elastic scattering of atmospheric and diffuse supernova background neutrinos) are expected in the region of interest for the planned exposure of DS-20k. DS-20k will have ultra-low backgrounds, with the ability to measure its backgrounds in situ, and sensitivity to WIMP-nucleon cross sections of 1.2 x 10^-47 cm2 (1.1 x 10^-46 cm2) for WIMPs of 1 TeV/c2 (10 TeV/c2) mass, to be achieved during a 5 yr run with exposure of 100 t yr.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.