I'm trying to identify a film I saw around 1968 or '69 but which was probably made several years earlier. It involved a visit to an underground civilization and just about the only scene I remember had the execution of a condemned prisoner, by chaining him to the floor (which may have had an image of the Sun) then opening the roof to expose him to the rays of the real sun, which rapidly turned him into a skeleton. Other than this, there was little or no explicit horror, so the film was suitable for children - I'd have been only nine or ten when I saw it. Also, unless I'm confusing it with a different film, the lead character, who was one of the visitors, was identified as being The Strongest Man in the World. Not much to go on, I know, but this site regularly works miracles!
This is Mole Men Against the Son of Hercules (1961).
Mole Men Against the Son of Hercules (Italian: Maciste, l'uomo più forte del mondo/ Maciste, the Strongest Man in the World) is a 1961 Italian peplum film directed by Antonio Leonviola and starring Mark Forest.
The young Princess Saliura has been captured by a fierce cave-dwelling people. Maciste and his friend Bangor, in whose care she was left, allow themselves to be also captured and imprisoned. They find themselves in a series of deep caverns, occupied by a race of mutant white-haired albino people. They are living under an ancient curse, by which they cannot emerge onto the surface, except by moonlight, and sunlight is deadly to them. They worship the goddess of the moon.
The 'mole men', led by Halis Mosab, a tyrannical and bloodthirsty non-albino queen, are using the captured people as slaves to operate their diamond mining operations. Saliura is assisted in an escape and is taken to sanctuary with the Guardians of the Sacred Waterfall. But she is betrayed and recaptured. Maciste and Bangor are forced to fight each other and then a ferocious ape in order to save the life of the princess, but she's condemned to be sacrificed anyway. Maciste not only survives, but escapes and saves Bangor and Saliura. Trapped in the caverns, the now-freed slaves break out onto the surface, thanks to Maciste's strength. Halis Mosab, who has become aware that she was captured as a child and somehow became queen of the 'mole men', is overjoyed when she comes to the surface and finds that she can live in the sunlight. But she accidentally falls off a cliff and into the waters of the Sacred Waterfall. The fate of her people is not revealed.
There's a scene where one of the albino men is sentenced to death by a queen, shackled to the floor (over a symbol of the Sun), then exposed to sunlight (from a window in the ceiling), which reduces him to a skeleton. You can view it at around the 46:14 mark in the video below.
Here's the Wikipedia description for the main character, Maciste:
Maciste (Italian pronunciation: [maˈtʃiste]) is one of the oldest recurring characters of cinema, created by Gabriele d'Annunzio and Giovanni Pastrone. He is featured throughout the history of the cinema of Italy from the 1910s to the mid-1960s.
He is usually depicted as a Hercules-like figure, utilizing his massive strength to achieve heroic feats that ordinary men cannot. Many of the 1960s Italian movies featuring Maciste were retitled in other countries, substituting more popular names in the titles (such as Hercules, Goliath or Samson).
I am thinking it is this: https://en.wikipedia.org/wiki/The_Mole_People and you might be mixing it up with another film which also involved a lost civilization which is of course a very common theme.
This Mole People film had a pre-Leave it to Beaver Hugh Beaumont and iirc there was some sort of extreme sensitivity to sunlight because the people lived underground. The film is from 1956.