Lambfellsgjá is a deep crack in the lavafield due to an earthquake. It's at the end of a very bumpy road, unless you have a big car expect about 50 min to drive here from the main road. From the parking lot it's about 20min walk through a lava field to get to the actual crack. If you go here, make sure to check out Trölladyngja as well as it's close by.