After two decades of intense research on high-temperature (high-T_c) superconductors, there is no consensus on the basic question: what is the mechanism that causes electron pairing? The ‘high-T_c problem’ remains one of the most important outstanding problems in condensed matter physics today. Even as we explore the possibility that pairing in these unconventional superconductors proceeds without the involvement of a bosonic glue, we have recently made exciting progress in identifying candidates that could potentially mediate pairing. I will present low temperature scanning tunneling microscopy data on the high-T_c superconductor Pr0.88LaCe0.12CuO4 (PLCCO) where we have discovered a bosonic mode at energies of 10.5±2.5 meV. I will discuss the possible origins of this mode and its implications for the pairing mechanism of the high-T_c superconductors.