Do we still need Expos?
While World Expos still showcase future technologies, some argue that the advent of the internet, mass media and cheaper foreign travel have made them redundant.
Global turmoil in the form of conflicts and trade wars has also led critics to question the idealistic values of unity and development touted at the events.
But organizers of Expo 2025 in Osaka have stressed that in-person exchanges between nations and the resulting "unexpected encounters" are still important.
Middle school teacher Yusuke Nagasawa said attending was a "valuable learning experience, to be able to actually experience the realism and warmth of the people, which cannot be conveyed through a screen."
"I've seen the excitement, and people from various countries have approached me for chats," added Nagasawa, who plans to bring about 140 pupils to the Expo next month.
Among the dizzying number of displays this year are a meteorite from Mars, a beating "heart" grown from stem cells, and the world's largest wooden architectural structure.
Where are they held?
Since 1928, the Paris-based International Exhibitions Bureau has run the Expos. More than 180 countries are members and the host city is chosen by a vote of its general assembly.
This is Osaka's second World Expo after the 1970 edition — featuring a Moon rock — that was attended by 64 million people, a record until Shanghai in 2010.
The United States once held frequent World's Fairs, as they are known there, leaving behind landmarks such as the Space Needle in Seattle and New York City's Unisphere.
But the world's largest economy last hosted one in 1984, with some experts saying their popularity has been overtaken by the Olympic Games and attractions such as Disneyland.