While several cities lay claim to the biblical Daniel's last resting place, within Iran both Jews and Muslims agree that this ornate Khuzestani ourchin dome (pine-cone-shaped tower) marks the spot. Pilgrims come from across the country to kiss the zarih (mesh grate) inside the gilded interior of what is a traditionally mosaic-clad imamzadeh (shrine). The current structure dates from 1871, though the tradition surrounding Daniel's relics is over 1000 years old.
In the Bible, Daniel's claim to fame was surviving the lions' den in Babylon, though there's no mention of his burial. The first accounts placing his remains in Susa (Shush) pop up around the 12th century, and proximity to the relics was thought to bring health and good fortune. With an influx of lucrative pilgrims, this caused jealousy on the part of the less fortunate on the other side of the river, so the grave was shunted back and forth between the two sides on alternate years. Eventually someone decided to lash it to the bridge in between.