Here is a link to lecture given by William Rodriguez who tells about his experience of feeling and hearing two separate explosions or some seven seconds apart as he was in the B1 level of the basement of the north Tower on 9/11. He was one of 14 of his co-workers who report witnessing an explosion that lifted upward the floor they were standing on at the B1 level and they assumed this explosion occurred at one of the six basement levels below them. What is important about their statements, is that this first explosion occurred some seven seconds before they heard a second loud noise, accompanying with vibration and building shake. They later realized that this second event was the impact of the commercial airplane.
You have to listen to William Rodriguez a second time, because it is hard for him to explain all of what he experienced that day. This is because he has experienced a very tramatic event and he just barely escaped getting killed
Here is another video tape of an eyewitness who tells his story about being in the B4 level of the basement of the Twin Towers on 9/11. And explosion that felt like an earthquake threw him to the floor. He helped a person who had a broken leg to get out of the basement and saw a dead person there as well. One of his co-workers had a wall smash into his face as the walls in the basement moved during this basement explosion. Again these injuries were in the basement levels , not up at the 84th
floor where the commercial airplane hit. He had to run in the basement that is connected under both the north and south Towers.
John Gross lead NIST engineer and investigator of 9/11 lies about melted steel