Here, in my opinion, is the very best Una furtiva lagrima ever recorded. Archivist Tom Frøkjær says it's a digitally remastered version of the romanza from Act II, Scene 2 of L'elisir d'amore, The Elixir of Love, by Gaetano Donizetti.
Caruso created the role of Nemorino the first time in February 1901 at La Scala in Milan. The conductor was the famous Toscanini, but the notoriously critical and reserved audience of La Scala did not react at all ... until Caruso started singing - and he basically turned the performance from failure to success. The audience went wild and Caruso was compelled to repeat "Una furtiva lagrima" three times.
When Toscanini came back stage to go before the curtain with the artists, he embraced Caruso, then turned to the manager of La Scala, Giulio Gatti-Casazza, and said: "Per dio! Se questo Napoletano continua a cantare così, farà parlare di sè il mondo intero." (By Heaven! If this Neapolitan continues to sing like this, he will make the whole world talk about him.)
And a few years later the whole world talked about him. - and still haven't forgotten his "velvet" voice and intense empathy.
Tom Frøkjær's non-profit website has more Caruso recordings and info. http://www.enricocaruso.dk