The N1 was a super heavy-lift launch vehicle intended to deliver payloads beyond low Earth orbit. The N1 was the Soviet counterpart to the US Saturn V, planned for crewed travel to the Moon and beyond. However, each of the four attempts to launch an N1 failed in flight, with the second attempt resulting in the vehicle crashing back onto its launch pad shortly after liftoff. The project was derailed by the death of its chief designer Sergei Korolev in 1966; the program was suspended in 1974 and canceled in 1976.
Video of explosion and more details: https://www.historyandheadlines.com/july-3-1969-largest-rocket-explosion-history-soviet-n1/
Iirc, by evaluation of aerial photographs after that incident, the Americans found out that the USSR's space program obviously didn't go according to plan.