In 1991 it was widely reported that the slippery goo had caused over 1,800 deaths and countless other injuries. So what’s the real cause of La Mancha Negra? Further lacking anyone bothering to study the substance, the two most plausible causes of La Mancha Negra are, first, that it’s just natural asphalt/bitumen seepage. This same phenomenon is commonly seen with improperly mixed asphalt, generally when too much asphalt is mixed in with the aggregate. There seem to be no further direct reports after this or other indication that La Mancha Negra still exists along the road in question today.