Hang Toi, widely called Dark Cave, is one of the adventure-oriented cave visits in the Phong Nha-Ke Bang area. Unlike the more show-cave style stops, it is usually experienced through a combination of zipline approach, swimming, helmeted cave walking, and a muddy interior chamber.
It is a good fit for active travelers who want a playful half-day rather than a purely scenic cave tour. Expect to get wet and dirty, and check current safety rules and water conditions before booking.