Spending 24 Hours Straight Under Water Challenge is a video created by MrBeast. In this video, Jimmy tries his best to sit under the pool water for 24 hours. This doesn't mean that he consistently held his breath underwater for 24 hours (that's scientifically impossible for a human), he is inside the water of the pool for 24 hours, and he was given support to stay under the water to not be afloat and support for breathing while doing this challenge (an upside-down bucket with air inside allowed Jimmy to breathe).
"I CAN'T BELIEVE I ACTUALLY DID THIS!!!!!!!" ― Official video description
It was hard for Jimmy to withstand being in there for 24 hours (the challenge started at daylight, then ended at around 11 PM) because Jimmy had the sudden urge to throw up and felt really seasick because of the water moving, so he had to end the challenge early for his health.
The thumbnail shows Jimmy sitting on the pool floor underwater while trying to hold his breath.