"It was finished in 139 CE, a year after the emperor's death, by his successor Antoninus Pious. [Hadrian's ashes were then transferred there from their temporary burial-place in the former villa of Cicero at Puteoli] It was then used up to the year 217 CE as a sepulchral for the Antonine family. Although constructed on the river edge, it was built on very solid ground and in an area previously used as a cemetery (actually in the gardens of Domitia, which, with those of Agrippina, formed a crown property called by Tacitus "Nero's Gardens.")
The monument was composed of at least three overlaid architectural bodies - a square base, a large cylindrical body, and a third and last piece. The latter was cylindrical too, but smaller in diameter and consisted of two overlaid orders, on the top of which the statue of Hadrian pulling a quadriga (chariot drawn by four horses) stood." - Castle.org
The structure was once topped with a tumulus of earth planted with trees. A ring of decorative marble statues stood along the edges of the parapet. I wish it still had the trees there. It would give the structure a less military aspect.
"Hadrian's ashes were placed here a year after his death in Baiae in 138, together with those of his wife Sabina, and his first adopted son, Lucius Aelius, who also died in 138. Following this, the remains of succeeding emperors were also placed here, the last recorded deposition being Caracalla in 217. The urns containing these ashes were probably placed in what is now known as the Treasury room deep within the building." - Wikipedia
The mausoleum of Augustus had last been opened to receive the remains of Nerva, but was no longer in use; so the Antonine emperors and their families were buried in the mausoleum of Hadrian, so it was later referred to by the name of Antoninorum sepulcrum.
Inscriptions actually recorded included the dedicatory inscription to Hadrian and Sabina set up in 139 A.D. (the latter was already deified, the former not) by Antoninus Pius, the sepulchral inscriptions of Antoninus Pius and Faustina, and of three of their children; of Aelius Caesar; of three children of Marcus Aurelius; of Lucius Verus, and of Commodus. That Marcus Aurelius himself was buried here is recorded by Herodian 4.1.4 who also relates that the urn containing the ashes of Septimius Severus and probably Faustina the younger were also interred here. Cassius Dio tells us that, besides Severus, Julia Domna, Caracalla and Geta were also laid to rest here. - edited from entry in Lacus Curtius.
"Much of the tomb contents and decoration has been lost since the building's conversion into a military fortress in 401 and inclusion by Flavius Augustus Honorius in the Aurelian Walls. The urns and ashes were scattered by Visigoth looters in Alaric's sack of Rome in 410, and the original decorative bronze and stone statuary was thrown down upon the attacking Goths when they besieged Rome in 537, as recounted by Procopius. " - Wikipedia