June 23rd, 1870, Mirza Mihdi (the Purest Branch, Baha'u'llah's youngest son) falls through a skylight in the prison. Baha'u'llah offers to heal him but Mirza Mihdi instead offers his life as a sacrifice so that those who desire may undertake pilgrimages. Baha'u'llah accepts and the Purest Branch dies at age 22. 70 years later, his body is entombed in the Monument Gardens on Mount Carmel.
Bahá’u’lláh referred to Mírzá Mihdí on the day of his death as "he that was created by the light of Bahá" and described his dying "at a time when he lay imprisoned at the hands of his enemies" as a "martyrdom." In the same tablet, Bahá’u’lláh extols Mírzá Mihdí’s station: "Happy art thou in that thou hast been faithful to the Covenant of God and His Testament, until Thou didst sacrifice thyself before the face of thy Lord, the Almighty, the Unconstrained. Thou, in truth, hast been wronged, and to this testifieth the Beauty of Him, the Self-Subsisting. Thou didst, in the first days of thy life, bear that which hath caused all things to groan, and made every pillar to tremble. Happy is the one that remembereth thee, and draweth nigh, through thee, unto God, the Creator of the Morn."
In December 1939—despite obstacles caused by the outbreak of World War II, local instability, riots, and the opposition of adversaries—Shoghi Effendi succeeded in transferring the remains of Mírzá Mihdí and Navváb from two different Muslim cemeteries in Acre to Mount Carmel in Haifa. On 24 December 1939 the coffins lay in state in the Shrine of the Báb. The following day, they were buried in the monument gardens near the Shrine of the Báb, alongside the resting place of Bahíyyih Khánum, Mírzá Mihdí’s sister, who died in Haifa in 1932.
More info: http://www.mirzamihdi.org/home