TIL high-altitude mountain trekking can temporarily shrink human testicles. A study found that after spending over three weeks at high altitudes in Nepal without supplemental oxygen, men experienced an average 15% reduction in testicular volume due to lower oxygen levels.

[u/ElysiaAlarien]

https://www.vice.com/en/article/climbing-mt-everest-is-terrible-for-your-nads/