Metallic glasses (MGs) provide ideal model systems for investigating amorphous physics including the secrets of glass- forming ability (GFA). Many MGs were reported to exhibit an anomalous exothermal peak (AEP) in the supercooled liquid region, indicating the occurrence of a hidden poly-amorphous phase transition. The structure origin of the AEP has puzzled the metallic glass community over four decades. Recently, the development of cutting-edge scattering facilities in the world such as neutron and synchrotron sources provide opportunities to reveal the origin of the AEP using a suite of bulk, in-situ, non-destructive structure probes covering multiple time and length scales. The latest results were found that Pd-Ni-P MGs, a prototypical glass with an AEP, underwent a reentrant supercooled liquid transition at a critical temperature. The origin was determined to be a liquid-liquid phase transition mediated by structure evolution at a medium-range length scale. We demonstrated that proper heat treatments could manipulate the microstructure of the MGs of an AEP. We also summarized the thermophysical parameters of several typical MGs with an AEP. It was suggested that the AEP might correlate with the GFA, which sheds light on exploring new amorphous alloys.