No, your eyes aren't playing tricks on you. Olympic swimmers are indeed wearing swim caps under their swim caps. This two-cap method isn't new. Michael Phelps, Katie Ledecky and more have been donning two caps for years.
The first reason might be the most obvious. Two caps help secure a swimmer's goggles, which usually go on the outside of the first cap and beneath the second.
The second reason, however, might be the most important. Two caps, including an inner latex cap and an outer silicone one, has a slight performance-enhancing effect.
"The outer silicone cap better maintains the shape and does not wrinkle as much [as the inner latex cap], thereby causing less drag," former women's Olympic assistant coach Dave Salo told Yahoo Sports during the 2012 Games in London.
Latex caps stick to the head better, while the silicone variety can slip off, as it did off the head of U.S. swimmer Dana Vollmer during the 100-meter butterfly final at the London Games.
Considering Vollmer still won the gold - even with just the one cap - it's clear that the performance-enhancing effects of two caps only go so far.