Terra Clues is a very intriguing online geography game. It basically has a collection of scavenger hunts with clues about places the player needs to find on an online map.
A number of the “hunts” available on the regular page may be too difficult for English Language Learners, primarily because of language. However, they have recently created a separate section for teachers and students that is divided by grade level. In this section, a teacher can create his/her own “classroom” that would have “hunt’s devised by the teacher.
And, of course, students could also create their own hunts.