Last updated: July 16, 2026

Quick Summary: Bioluminescent plankton glow in the water when disturbed by a paddle or a passing fish, and Lan Ha Bay near Cat Ba is one of the best places in northern Vietnam to see it on a night kayak. Sightings are most likely on dark, moonless nights, and never guaranteed.

Last updated: July 2026

Halong Bay and Lan Ha Bay sit on the same stretch of coastline, separated by an administrative border rather than anything you could see from a boat. The limestone towers are the same rock, the water is the same colour, and the two bays are close enough that a fast vessel crosses between them in under an hour.

Ask ten travel agents which one to book and you will hear ten versions of the same line: Lan Ha is the quiet one. That is broadly true today. It is also becoming less true each season, and very few operators will explain why.