Add a new layer below labels
Using the second argument of addLayer, you can be more precise
<!DOCTYPE html>
    <meta charset='utf-8' />
    <meta name='viewport' content='initial-scale=1,maximum-scale=1,user-scalable=no' />
    <script src=''></script>
    <link href='' rel='stylesheet' />
        body { margin:0; padding:0; }
        #map { position:absolute; top:0; bottom:0; width:100%; }

<div id='map'></div>
mapboxgl.accessToken = '<your access token here>';
var map = new mapboxgl.Map({
    container: 'map',
    style: 'mapbox://styles/mapbox/streets-v9',
    center: [-88.13734351262877, 35.137451890638886],
    zoom: 4

map.on('load', function () {

        'id': 'urban-areas-fill',
        'type': 'fill',
        'source': {
            'type': 'geojson',
            'data': ''
        'layout': {},
        'paint': {
            'fill-color': '#f08',
            'fill-opacity': 0.4
    // This is the important part of this example: the addLayer
    // method takes 2 arguments: the layer as an object, and a string
    // representing another layer's name. if the other layer
    // exists in the stylesheet already, the new layer will be positioned
    // right before that layer in the stack, making it possible to put
    // 'overlays' anywhere in the layer stack.
    }, 'water');

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