Well it does happen somewhat regularly. People (often from poorer countries) get on planes as stowaways. Most times they can only get into the wheel well, sometimes cargo holds. But they dont know that these non-passenger areas often aren’t heated or pressurized and either freeze to death or suffocate during high altitude flight. Getting crushed in the wheel well during take off or falling out it during landing can also be deadly.

Stowaways often get in there. It doesn’t typically end well for them. That area is not pressurised or heated.

https://en.m.wikipedia.org/wiki/Wheel-well_stowaway