I present the result of an observational campaign to map the 21 cm atomic hydrogen line in host galaxies of supernovae (SNe), long gamma-ray bursts (GRBs), and fast radio bursts (FRBs). For all analysed hosts of type Ic-BL SNe and GRBs we found off-centre gas concentrations close to the GRB/SN positions and irregular velocity fields. This suggests a recent gas inflow. This is consistent with GRB/SN progenitors being born when a galaxy accretes metal-poor gas from the intergalactic medium, and opens a possibility to use GRB/SN hosts to study gas accretion. This supports a very massive model for their progenitors. Similarly, the host galaxies of FRBs exhibit very asymmetric 21 cm lines, suggesting a connection between the birth of FRB progenitors and galaxy interactions. On the other hand, the host galaxy of the unusual transient AT 2018cow (the first fast blue optical transient, FBOT) does not exhibit such features, which suggests that its progenitor may not have been a massive star.

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