Wednesday marked a day of remembrance for those who lost their lives in the Monongah Mine Disaster, an event that’s widely considered to be the worst mine disaster in U.S. history. On Dec. 6, 1907, Monongah mines number six, and number eight suffered a large explosion that killed 361 miners.

Although the explosion’s exact cause was never discovered, some theorized that it was caused by the ignition of accumulated dust and gas in the air. To commemorate their lives, a candlelighting ceremony and blessing of the graves was held at the Mt. Cavalry Cemetery alongside the placing of a wreath.