Ha Long Bay is situated on the north east coast of Vietnam, a ride of about 160 kms from Hanoi. It is an area of about 1500 square kilometres comprising some 2000 islets and Karsts, which has been designated a UNESCO Heritage Site. Ha Long Bay means the bay of descending dragons. It has its own micro ecology and bio diversity and a huge variety of islets.
You can access the bay from a number of places but the most obvious are Halong town and Bai Chai a small town about 15 minutes away. Travel around the islands is by boat, either old junks which cruise slowly around and on which you can stay overnight or by smaller boats on day trips. One of the delights of this trip is the Vietnamese food that is made locally especially the sea food. On the boats you can order food and have a great meal as you cruise around. The nice thing is that you can order so you get something special.
There are lots of people selling tours to Ha Long in Hanoi and the local area, many acting only as agents and selling tickets and those tickets can have various prices for the same tour or trip, so it is important to look around. The price differences can be big. For myself I negotiated the various elements of the visit independently so got amazing deals on transport boats and food, but tat is easier if you are a solo traveller than if in a group. Also I stayed in the town of Bai Chai rather than Halong itself , although very close by and that town is very good value. It is being developed for the mass market, so as with many places in Vietnam it is going to change quickly.
The islets Karsts and the seas around them are a visual delight, something quite unique, but 2000 islets is a lot so its good to be selective and visit the ones you want. Each has a name, the elephant, chicken, he and she, etc so you can be specific. There are many caves and grottos on the islets, some can only be visited at low tide and others any time so find out about those. Some, unfortunately in my view, have lighting that can give them a slightly Disneyworld feel, but they are amazing for all that.
Halong is a joy, and an indispensible part of any tour of the variety and wonders of North East Vietnam.