Dragon Burial Mounds

Dotted across the fells and forests, the plains and snowlines of Skyrim are strange circular mounds, surrounded by a scattering of standing stones.

These are actually ancient dragon burial mounds; the final resting place of these creatures when they were slain centuries ago.

Alduin the World-Eater has returned to Skyrim, and over the course of the Main Quest, he opens these mounds and resurrects the dragons within, calling them forth to wreak havoc once more.

How Dragon Mounds open
There are 22 Dragon Mounds scattered throughout the nine Holds of Skyrim. When you encounter one (all are listed in the Atlas, and marked on the Hold maps), it will be in one of four states. Over the course of the Main Quest, these mounds gradually open, releasing the dragons trapped within.

STATE I: DORMANT

Each Dragon Mound opens in response to a specific objective in the Main Quest. Before that point, the Dragon Mound is dormant. The ground is covered, and the site could easily be mistaken for an ancient Nordic burial mound, as the two are quite similar.

STATE II: DESERTED 

As the Main Quest develops, you may find some mounds that have been opened, but have no dragon nearby. That is, Alduin has visited this site, resurrected a dragon from the mound, and both creatures have flown away.

STATE III: AWAKENED

As the Main Quest goes on, you will find more and more mounds in this state. Alduin has visited the site, resurrected the dragon from the mound, and flown away. Meanwhile, the reborn dragon remains here, gathering its strength and waiting for a chance to strike.

STATE IV: RESURRECTION 

And in a few cases, if you stumble across just the right mound at just the right time, you may encounter Alduin himself. If you watch, Alduin will resurrect the dragon before your eyes, then fly off to his next destination. Attack, and Alduin roars into the skies before making his escape (he cannot be harmed). Meanwhile, the resurrected dragon will turn and attack. If you miss this opportunity, the mound will change to State III, with the newly-resurrected dragon remaining near its mound until you arrive to challenge it.