That durability is the result of intensive testing, including one peculiar late night revelation. "Even a tiny hole, say 2millimeters, can take weeks off the lifespan of the balloon. And trying to find a 2mm hole on a balloon with a surface area of 600 square meters is like looking for a needle in a haystack," says Krishnaswamy. "I was trying to figure out ways to test for leaks, and suddenly, in the middle of the night, it came to me. Condoms!"
Along with The Marathoner, Project Loon is highlighting a few other notable balloons. The Sprint Star title goes to the fastest Loon balloon, which traveled 324 kilometers per hour while rushing to the South Pacific Ocean over Antarctica. The Frosty Survivor endured temperatures of -83°C (-117°F). "The weather is tough, because the balloons can get very brittle," says Krishnaswamy. Rapid temperature shifts as the balloon ascends and descends cause it to contract and expand. "Imagine a rubber band you put in the deep freeze and then try to stretch. Snap!"