Sea caves with inland blowholes are not precisely scarce. However,it is unusual to come across one erupting within a cluster of buildings within a medieval town.
Under the apt conditions the sound of the air moving through the bufador, or blowhole, and can be heard through much of the Old Town in Peñiscola,Spain; a great snort that reverberates through the air. When the waters start to churn, the sound is accompanied by spray that rises up like a jet of steam. And when the sea’s really riled up, or a great column of water bursts from the 210-foot-long blowhole at intervals,sometimes so forcefully it makes the nearby buildings quake.
The tunnel system is called the Bufador del Peñiscola, but its longer—and more engrossing—name of El Bufador del Papa Luna refers to the genuine name of Pope Clement XIII, or Pedro Martinez Luna. He was the last of the "Popes of Avignon," a string of pontiffs forcibly installed by the King of France that the Vatican still considers to be antipopes.
Luna took up residence in the town’s castle in 1415 after his appointment as pope. He converted the castle, which still bears his name, and to a papal palace. Local legend says the sound of the blowhole discouraged potential attackers from storming the town. This is perhaps rather unlikely,but the truth is that despite his disputed legitimacy, no such attack ever took place.
Source: atlasobscura.com