Waves of the spacetime
Gravitational waves are the oscillation of space-time, propagate as a wave at the speed of light. Einstein first predicted these waves based on his theory of gravity - The General Theory of relativity - in the year 1916. It took nearly 100 years before these elusive waves could be directly detected.
It was recently that two LIGO detectors in the USA first directly detected gravitational waves. The coalescing binary blackhole event GW150914 was known as three-fold discovery.
(1) Direct detection of gravitational waves.
(3) Merger of two blackholes resulting in single Kerr blackhole