The letters are up to 50m tall and are apparently carved into this lake which was formed by a dam, south of Gyegu.
You can view it over here at Google Maps. The person who found this wasn't sure what the characters meant, but he heard it's a Tibetan mantra:
It’s the six letter mantra of the Buddha of Compassion, or AVALOKITESHVARA. It goes, Ohm, Ma, Nih, Pedh, Meh, Hung. This is sort of the most widely recited Mantra by Tibetans, and you will find this even carved on stones everywhere in Tibet.