Significance When compressed above megabar pressures (100 GPa), glasses may undergo structural transitions into more densely packed networks that differ from those at ambient pressure. While inelastic X-ray scattering (IXS) provides a rare opportunity to probe the pressure-induced bonding transitions, a decade of efforts to collect an IXS signal from any matters beyond 100 GPa have not been successful. Here, IXS spectra for B ₂ O ₃ glasses up to ∼120 GPa revealed its unique densification paths characterized with the unexpected stability of four-coordinated boron ( ^[4] B). This is in contrast to other prototypical glasses where highly coordinated cations ( ^[4,5,6] Si and ^[4,5,6] Ge) form at much lower pressure, confirming that the cation with a smaller atomic radius undergoes coordination transformation at higher pressure.