These trenches have a slope of ~10-15 degrees on the steep side (and ~5 degrees on the gentle side). So it would go down 1mi for every 4mi further from the shor

These trenches have a slope of ~10-15 degrees on the steep side (and ~5 degrees on the gentle side). So it would go down 1mi for every 4mi further from the shore. To get to 4mi deep it only needs 12mi of horizontal distance in the steepest of cases. Even the Mariana Trench is imagined by most people looking like a canyon (almost vertical walls), when it actually looks more like sloped plains.