Answer: He referred to the spread of France's republicanism in Europe.
Step-by-step explanation:
When Hugo said that he hears “the dull sound of revolution, still deep down in the earth, pushing out under every kingdom in Europe its subterranean galleries from the central shaft of the mine which is Paris” (“Les Miserable”) he anticipated the revolution and spread of republicanism from France to whole Europe. The February Revolution in 1848. brought republicanism to France and soon to the Great Britain and all of the Europe countries.