A tiny version of Seljalandsfoss or Kvernufoss. Another waterfall you can walk behind, but it hasn't really been discovered yet! There's actually two waterfalls here, and we're not sure which one is Gluggafoss and which one is Merkjárfoss, but they're very close together.
Coming from Reykjavik on the 1 Road, turn onto 261 Road in Hvolsvöllur. If you're coming from the direction of Vik and don't have a 4x4 avoid the 250 Road, it won't do your car any good. Keep following the 1 Road until you can turn right onto 261 Road in Hvolsvöllur. Of course, if you have a 4x4, you can take the 250 Road.
After you've parked just simply follow the stream up for about 125 meters to the first waterfall, this is the one you can walk under. Walk a bit further north to the second waterfall, it's no more than 50 meters further.