  If Dark Matter is made of Weakly Interacting Massive Particles (WIMPs) with
masses below $\sim$20 GeV, the corresponding nuclear recoils in mainstream WIMP
experiments are of energies too close, or below, the experimental threshold.
Gas Time Projection Chambers (TPCs) can be operated with a variety of target
elements, offer good tracking capabilities and, on account of the amplification
in gas, very low thresholds are achievable. Recent advances in electronics and
in novel radiopure TPC readouts, especially micro-mesh gas structure
(Micromegas), are improving the scalability and low-background prospects of
gaseous TPCs. Here we present TREX-DM, a prototype to test the concept of a
Micromegas-based TPC to search for low-mass WIMPs. The detector is designed to
host an active mass of $\sim$0.300 kg of Ar at 10 bar, or alternatively
$\sim$0.160 kg of Ne at 10 bar, with an energy threshold below 0.4 keVee, and
is fully built with radiopure materials. We will describe the detector in
detail, the results from the commissioning phase on surface, as well as a
preliminary background model. The anticipated sensitivity of this technique may
go beyond current experimental limits for WIMPs of masses of 2-8 GeV.
