If your horse is unwilling to go forward, get off and lead him or give him to your companion while you check the ground on foot. Remember if he is adamant, he may know best. On grass hills look for straw coloured grass and avoid areas with red tinged grass which grows on boggy ground. Follow sheep tracks through reeds as sheep know the best lines to follow. Avoid bright green mossy areas as they tend to be bottomless.
Cotton grass is another good indicator of unsuitable ground.
Should your horse sink in, even a little, get off at once and encourage him to extricate himself. If he is in deep, he may lie still in shock. Loosen the girth if you can, wait and then encourage him again, very strongly if necessary. These remote areas are often flown over by low flying jets. But it is often the rider rather than the horse that is upset by their sudden noisy appearance.