An Italian couple and their 11-year-old son were killed when a hole opened up in the ground at a bubbling volcanic crater near Naples — a popular tourist site.

Key points: The victims were a 45-year-old man, a 42-year-old woman and their son

The victims were a 45-year-old man, a 42-year-old woman and their son "Either there was a small explosion, or the ground simply gave way from their weight," spokesperson said

"Either there was a small explosion, or the ground simply gave way from their weight," spokesperson said The Solfatara volcano is one of many volcanic craters in the Campi Flegrei area

Italy's fire brigade said it pulled three dead bodies from a shallow hole in a section of the crater at the Solfatara volcano, which was surrounded by a wooden fence meant to keep tourists out.

The victims were a 45-year-old man, a 42-year-old woman and their son. Another son, aged seven, did not fall into the hole and was unharmed, national fire brigade spokesman Luca Cari said.

"Either there was a small explosion, or the ground simply gave way from their weight, and they fell into this hole," Mr Cari said.

"It was inside a fenced-off area."

Firemen on the scene said the three victims, who were on holiday from northern Italy, appeared to have died from asphyxia, possibly because of hot gases emanating from the ground. The official cause of death will be determined by an autopsy.

The Solfatara volcano is one of many volcanic craters in the Campi Flegrei area, 20 kilometres west of Naples, which first opened up to tourists in 1900, according to its website.

The accident happened at the Bocca Grande (Big Mouth), the largest of the fumaroles in the area, which the ancient Romans called the home of the god of fire.

The ground at the site emits water vapours — of 160 degrees Celsius — and gases, including poisonous hydrogen sulphide, the website said.

The accident happened at the Bocca Grande, the largest of the fumaroles in the area. ( Wikimedia: Patrick Massot )

Reuters