Where does the story The Twenty-One Balloons take place?

The Twenty-One Balloons, by William Pene Dubois, opens in San Francisco, California, in October, 1883, as Professor Sherman makes a triumphant return to the city where he had launched his year of what he had planned to be a solitary and peaceful balloon voyage. As a returning hero, he tells the story of his voyage first to the Western American Explorers' Club, which he is an honorary member of.

Professor Sherman, who has just retired from forty years of teaching math to schoolboys, had set off from San Francisco in his balloon over the Pacific Ocean, intending to let the winds blow him where they wished for one year. But as he relates his story to the Club, it is clear that his voyage has not gone as planned.

A series of unfortunate and fortunate events lead him to crash-land right off the coast of Krakatoa, close enough for him to drag himself out of the water and onto the shore.  Krakatoa is an island that is part of Indonesia, a country between the Indian and Pacific Oceans, south of the Asian continent and north of Australia, a real place with a real volcano that did erupt violently in August, 1883.

Professor Sherman finds the island to be occupied by twenty families, who hospitably take him in, and so it appears that he is going to settle there. However, when the volcano erupts, all the families and Professor Sherman must flee the island with the help of a balloon-driven raft. The families drop off in various destinations as they fly over India and Persia (Iran), and then a large swathe of Europe. Professor Sherman hangs on so he can ditch the rescue raft and balloons, which he is able to do in the Atlantic Ocean, where he is picked up by a rescue ship on September 8, 1883.

Professor Sherman then proceeds to cross the United States by train, arriving in San Francisco, which is where the story opened and where his voyage began.