It's important to add that even the black box from the TWA flight that crashed in the ocean was recovered. These flight recorders are made of steel or titanium, build to withstand temperatures in excess of 2,000' F, painted bright orange, and emit signals which are detectable with special recovery equipment. One 48 year aircraft accident investigator was famously quoted as saying that he had never in his entire career responded to an accident wherein the box was not recovered. A volunteer at Ground Zero claimed to have personally witnessed the recovery of one box and later testified before the 9/11 Commission, but was not included or even mentioned in the report. He claimed that FBI agents recovered at least one, but possibly as many as three boxes (there are 2 in each plane) from the WTC. And they wonder why people start conspiracy theories.