A 15-foot wall of water could roll across Grand Isle if a landslide occurred in the Mississippi Canyon, a trench in the Gulf of Mexico floor about 30 miles off the mouth of the Mississippi River. And unlike a hurricane, residents would have just an hour's notice – not days. Such landslides have happened about once every 1,000 years in that area, and that time frame is almost up. Although being a low probability event — one in a thousand — it still is a credible event, and would be of a high impact. The National Weather Service has found evidence of previous slides in the canyon, which could cause significant tsunamis. The material falling into the canyon would displace the water that is already there, and that would cause the wave. It wouldn't be very noticeable in the open Gulf, but as the wave reached the shallower water near the shore, it would rise up. The hypothetical landslide used in National Weather Service computer model was 13 miles wide, 40 miles long and dumped enough rocks