As the king of the gods and heir to Uranus, the Titan Cronus is a sky god, like his father and like his son after him. He controls all of those things in and under the sky and as such, Rhea has been unable to hide from him. Gaia, though, knows a trick. Cronus’ knowledge and power end at the ground and do not extend into the Underworld.
To the Ancient Greeks, the Underworld was a real place, and any time men went below the ground, in a cave or mine, they were treading in the Underworld. Today we would call such a place Hell, but for the Greeks, while it was strange and foreign to them, it was not a place of evil. All the dead dwelt there, as did monsters and prisoners of the gods, not just the evil.
It was thought to be a place guarded by strange and dangerous creatures though, so any of that time would have tread the dark, underground places with a certain amount of trepidation.
Gaia now uses this to her advantage to hide Rhea and her unborn child from Cronus’ prying eyes, for Cronus fears the Underworld as well, thanks to his encounter with Nyx at the dawn of the age.