The discovery of the Higgs boson added a fundamental piece to the Standard Model, but it also opened new questions about the nature of electroweak symmetry breaking and possible connections to hidden sectors. A broad class of models predicts new resonances below the Higgs mass, despite the mainstream interest in the high energy regime, such as light scalars/pseudoscalars, or dark mediators that could evade traditional search strategies. In this seminar, I will discuss the experimental challenges and novel techniques used in ATLAS to probe this low mass regime, where large Standard Model backgrounds and trigger limitations demand creative solutions. I will highlight a biased selection of searches and discuss ongoing efforts to extend sensitivity to unexplored regions of parameter space.