The study of the atmosphere and structure of temperate Earth-mass planets is a challenging but promising endeavour to bolster our scientific understanding of the diversity of small planets and their atmospheres. The unique Kepler-138 system is composed of at least three small (<2 Rearth) planets with measured transit timing variations (TTVs) yielding precise masses. In particular, the low density of Kepler-138d (Mp=1.2 ± 0.3 Mearth, Rp=1.57 ± 0.06 Rearth) makes it one of the most favorable small temperate targets for transmission spectroscopy. I will present a detailed study of the interior structure and atmosphere of Kepler-138d based on three newly-obtained spectroscopic transit observations using HST/WFC3 and Spitzer and a Keck/HIRES RV follow-up of Kepler-138. The precise transit times enable us to refine the mass estimates, while the transmission spectra give us the first insight into the atmosphere of such a low-mass temperate planet. We show that Kepler-138d’s bulk density and the precisely-measured flat transmission spectrum are consistent with a “water world” scenario without the need for a hydrogen atmosphere. The presence of a hydrogen-dominated atmosphere is only consistent with our observation if it is very thin (<0.3 % by mass) and, simultaneously, a thick deck of clouds or photochemical hazes blocks our spectroscopic view into the atmosphere. We argue that core-powered mass-loss would likely have eroded such a thin primordial atmosphere if the planet and atmosphere formed at its present orbital distance. Atmospheric escape could also have also removed such a thin H2 envelope over less than 1 Gyr.