  We present the conceptual design and the physics potential of DarkSPHERE, a
proposed 3 m in diameter spherical proportional counter electroformed
underground at the Boulby Underground Laboratory. This effort builds on the R&D
performed and experience acquired by the NEWS-G Collaboration. DarkSPHERE is
primarily designed to search for nuclear recoils from light dark matter in the
0.05--10 GeV mass range. Electroforming the spherical shell and the
implementation of a shield based on pure water ensures a background level below
0.01 dru. These, combined with the proposed helium-isobutane gas mixture, will
provide sensitivity to the spin-independent nucleon cross-section of $2\times
10^{-41} (2\times 10^{-43})$ cm$^2$ for a dark matter mass of $0.1 (1)$ GeV.
The use of a hydrogen-rich gas mixture with a natural abundance of $^{13}$C
provides sensitivity to spin-dependent nucleon cross-sections more than two
orders of magnitude below existing constraints for dark matter lighter than 1
GeV. The characteristics of the detector also make it suitable for searches of
other dark matter signatures, including scattering of MeV-scale dark matter
with electrons, and super-heavy dark matter with masses around the Planck scale
that leave extended ionisation tracks in the detector.
