PALMYRA, Syria — Explosions rocked the ancient town of Palmyra on Friday and on the horizon, black smoke wafted behind its majestic Roman ruins, as Syrian army experts carefully detonated hundreds of mines they say were planted by Islamic State militants before they fled the town. An Associated Press crew visiting the town Friday witnessed firsthand the destruction inflicted by the extremist group on the town’s famed archaeological site, less than a mile away from the modern-day town of the same name, now completely deserted. While some parts of the site, including the Roman-era grand colonnades and amphitheater appeared relatively untouched, the damage was very much visible elsewhere. The remarkable Arch of Triumph, built under the Roman emperor Septimius Severus between A.D.