What Caused the Fiery Ground-Level Explosions Right Before the Collapse of Each Tower?

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On 9/11, there were multiple reports by credible witnesses of ground-level fiery explosions right before the collapse of the Twin Towers:

• A CBS News reporter on stated: "All of a sudden I heard a roar and I saw one of the towers blow ... I saw from street level as though it exploded up, a giant rolling ball of flame..."

• The same reporter stated: "I hear simultaneously this roar and see what appears to be a gigantic fireball rising up at ground level . . . giant ball of fire come out of the earth" (pages 119 & 239)

• A Port Authority Police Department officer, who was intimately familiar with the World Trade Center from his years of police duties patrolling there, said that hallway began to shudder as a "terrible deafening roar" swept over him, then a giant fireball exploded in the street seconds before the south tower collapsed

• An eyewitness reported a large explosion at ground level right before the collapse of the North Tower

What could have caused ground-level fiery explosions right before the collapse of each tower?

In addition, a paramedic stated that before each tower collapsed, the ground shook (pp. 5-7). What could have caused the ground to shake?

See also this and this.