Ram-Pressure Stripping Caught in Action in a Forming Cluster at z $\sim$ 2.5

Galaxy clusters in the local Universe are dominated by massive quiescent galaxies with old ages, formed at high redshifts. It is debated whether their quenching is driven by internal processes or environmental effects, which has been challenging due to the lack of observations during their peak formation epoch. Here we report clear evidence from ALMA of extended and elongated gas tails in five galaxies in a forming cluster at z = 2.51. The single-tailed gas distributions extending significantly beyond the stellar emission probed by JWST, in galaxies that are rather isolated and without signatures of mergers or interactions, signatures highly uncommon in the field, provide evidence of ram-pressure stripping (RPS). These represent the most distant confirmed cases of RPS, highlighting the critical role of environmental effects in gas removal at high redshifts, an often overlooked quenching pathway.