Pulsar Timing Array (PTA) collaborations have recently announced strong evidence of nanohertz gravitational waves (GWs). These findings are enabled by the precise timing of millisecond pulsars, which reveals the characteristic signature of GWs embedded in the correlation amongst pulsar noise, known as the Hellings-Downs curve. This talk explores the principles of PTA, methods for GW signal identification, and the latest findings from the European PTA (EPTA) collaboration. Although some properties of the common correlated noise could be identified, its origin remains a mystery. This presentation delves into various interpretations, from supermassive black hole binaries to cosmological and early Universe sources.