A husband, wife and their 11-year-old son were all engulfed in a hole as the ground opened up under their feet.
Only the couple’s younger son, a boy of seven, survived the horror at the Solfatara volcano near the Italian city of Naples.
The tragedy, which left other visitors and site workers distraught, happened inside an area fenced off for safety.
Last night it was thought the boy had gone past the fencing and broken through the surface.
His dad is believed to have rushed to help him, only to fall in himself.
The mum then went to pull them out and died too.
Eyewitnesses reported arriving at the spot to find the younger boy sobbing for help.
Shocked Diego Vitagliano said: “I saw a child run crying. I did not think I was facing the worst tragedy of my life.”
Firefighters recovered all three bodies from the crater, which emits poison gases and steam at temperatures of 320F (160C).
Fire spokesman Luca Cari said: “Either there was a small explosion, or the ground simply gave way from their weight.”
The victims, named locally as Massimiliano Carrer, 45, wife Tiziana, 42, and son Lorenzo, from northern Italy, are thought to have been asphyxiated.
The crater-riddled area, known to the ancient Romans as the home of the god of fi re, was first opened to tourists in 1900.