Seven Yellow Balloons

In 1967 the Cold War had been going on, the Soviet Union and Cuba had diplomatic ties. On June 29, 1967, on the Florida coast near Hallandale, a crate was 18 inches and no bigger than a breadbox was spotted floating in the ocean. It was stamped "Property of U.S.S.R." and addressed to Institute of Mineral Resources of Cuba. Inside the crate was seven yellow balloons with black rubber stoppers.

It was indicated on the box that they were shipped from Leningrad, Russia and was supposed to go to Havana, Cuba. It's possible that the crates were in the water for at least 8 weeks.  The crates were listed as50 kg  (110 lbs)but they were only 13.6 kg (30 lbs). The air inside the balloons was originally believed to have been toxic. But it was found that there was no indication of the air being poisonous.

Another crate was found 217 km away in Marathon. The only difference in this crate was that it was empty. There were no balloons in it and the reason for the boxes to be shipped off this way is still a mystery. It's not believed that this was a hoax.

SOURCES:

http://www.weirduniverse.net/blog/comments/mystery_balloons/

https://www.thehits.co.nz/photos/the-yellow-cuban-balloons/

https://mysteriousfacts.com/3-historical-mysteries-you-wont-stop-thinking-about/