Tunguska was a site in Siberia on Earth, where there was a large explosion of mysterious provenance in the early 20th century. Hikaru Sulu described it as a great meteor impact that flattened entire forests. (TOS: "That Which Survives")
In 2022, Sera told La'an Noonien-Singh and an alternate version of James T. Kirk that Tunguska was actually an alien attack meant to impede Human progress. Ironically, Sera herself was not a conspiracy theorist but in fact a Romulan agent sent from the future with just that objective. (SNW: "Tomorrow and Tomorrow and Tomorrow")
In 2268, Sulu mentioned the Tunguska event to Captain James T. Kirk, with regard to the possibility that the USS Enterprise had impacted the planet they were investigating. Kirk dismissed the comment, stating that if he wanted a Russian history lesson, he would have brought Pavel Chekov. (TOS: "That Which Survives")