One of the most captivating experiences when cruising through the stunning seascape of Lan Ha Bay is exploring the Dark & Bright Cave area (Hang Sang Toi). This natural wonder isn't a dry cave for walking but rather a fascinating water cave system leading into a serene, enclosed lagoon. Accessing its hidden beauty is typically done via kayak or a traditional bamboo boat rowed by locals, making it an adventurous and rewarding activity often included as a highlight on Lan Ha Bay itineraries.

The name refers to two distinct passages through the karst formations. The 'Dark Cave' is a longer, unlit tunnel that requires careful navigation (sometimes with headlamps depending on tide and operator), offering a thrilling sense of mystery as you paddle through the enclosed space listening to dripping water. The 'Bright Cave' is a shorter, picturesque archway naturally illuminated, leading into the same tranquil lagoon. Emerging from either tunnel into the peaceful, lake-like lagoon, completely surrounded by towering, jungle-clad limestone cliffs, is a truly breathtaking moment.

Exploring Dark & Bright Cave allows you to appreciate Lan Ha Bay's intimate beauty and geological wonders up close. The calm waters within the lagoon are perfect for leisurely paddling, observing the lush vegetation clinging to the cliffs, and sometimes even spotting local wildlife like Cat Ba langurs high above. This combination of cave exploration and serene lagoon discovery makes Hang Sang Toi a quintessential and memorable activity, perfectly showcasing the adventurous yet peaceful spirit of cruising in Lan Ha Bay.