I learned to us what we called a "pit fire" as a kid living on the great flatlands of the mid-west. There is usually a rather brisk wind blowing and a normal "on the ground" fire will be very in-efficicient. Either two joined holes or a trench. Two holes works better in very strong winds as you can block off one hole to reduce the draft. Sometimes it is better to put the fire in the upwind hole, sometimes in the downwind hole. Depends on how hot a fire you want, how much wind there is and what type of fuel you are using.

Experiment with it a bit and you will find you can get a very hot fire with just dried grass or very small sticks. But be wary of underground fire as the fire may travel along the roots of grasses and pop up someplace else. As with any fire, be sure it is out before you leave it.