Rangko Cave is a sea cave northeast of Labuan Bajo with a clear blue saltwater pool inside. When the sun is high enough to enter the opening, the water glows an almost artificial blue, and swimming in the cool pool feels wonderfully strange after the hot road in.
Getting there takes effort: a drive to Rangko village, then a short local boat ride and a walk. Time the visit around midday for the best light, but avoid entering if the tide or local advice says conditions are unsafe. Bring cash for transport and local fees.