According to my trust Strategy Guide, that is Ghost Barrow: SPOILERS AHEAD!!!
Supposedly a ghostly figure can be seen moving towards this area at night. There may also be grave robbers in the area. The trapdoor is an Adept level lock and there is some loot to be had..... it's also unrelated to any quest so I have no idea why you can't get into it....
It's a triangle arch with a circle cut out, with 4 steps leading up/down from it, with stones standing along the sides of the steps.