Abstract: Exploration of unconventional superconductors and elucidation of their mechanism have been important frontiers in condensed matter physics for more than 40 years. Heavy fermi superconductors, copper oxide high-T_c superconductors, and iron-based superconductors are three large families of unconventional superconductors. Cuprates are the only family that hosts high superconducting transition temperatures above the boiling point of nitrogen at ambient pressure. Nickel oxides with Ni⁺ are expected to show unconventional superconductivity due to the similarity of the electronic configurations of Cu²⁺ and Ni⁺. Recently, we discovered superconductivity under 14 GPa at 80 K in a bilayer nickelate La₃Ni₂O₇ with an average valent state of Ni^2.5+. In this paper, we will describe the verification of superconductivity by electronic and magnetic measurements, as well as the determination of the crystal structure of La₃Ni₂O₇ under pressure. The obvious difference of the magnetic ground state of La₃Ni₂O₇ from that of copper oxide and iron-based superconductors may be vital for elucidating the mechanism of unconventional superconductivity.