Lan Ha Bay is the southern extension of the Halong Bay limestone karst, separated from the famous main bay by Cat Ba Island. It is a kilometres-wide maze of about 400 small karst islands and pinnacles, dozens of hidden beaches, and several floating fishing villages that still operate as working communities rather than tourist sets.
For visitors based on Cat Ba, Lan Ha is the obvious day-trip: a boat from Beo Wharf takes you out for kayaking through limestone tunnels, swimming off the boat in clear water, and exploring caves. Compared with Halong Bay proper, the islands are smaller, the water is cleaner, and the traffic is dramatically lighter. Half-day trips cost about half what an overnight Halong cruise does; a full day with kayaks is the sweet spot.