JSFiddle - React, Tailwind, and code Playground

HTML

<script src="https://openlayers.org/ol3-cesium/ol3cesium.js"></script>
<script src="https://openlayers.org/ol3-cesium/Cesium/Cesium.js"></script>
<link rel="stylesheet" href="https://openlayers.org/ol3-cesium/ol.css">
<div id="map2d" style="width:600px;height:400px;float:left;"></div>
<div id="map3d" style="width:600px;height:400px;float:left;position:relative;"></div>

JavaScript

var map = new ol.Map({
  layers: [
    new ol.layer.Tile({
      source: new ol.source.OSM()
    })
  ],
  target: 'map2d',
  view: new ol.View({
    center: [0, 0],
    zoom: 2.5
  })
});
map.getView().setCenter(ol.proj.transform([-20, 75], 'EPSG:4326', 'EPSG:3857'));

var geometry = new ol.geom.MultiPolygon([ 
    [ [ [-30, 70], [  0, 70], [  0, 69], [-30, 69], [-30, 70] ] ],
    [ [ [-40, 71], [ 10, 71], [ 10, 70], [-40, 70], [-40, 71] ] ],
    [ [ [-50, 72], [ 20, 72], [ 20, 71], [-50, 71], [-50, 72] ] ],
    [ [ [-60, 73], [ 30, 73], [ 30, 72], [-60, 72], [-60, 73] ] ],
    [ [ [-70, 74], [ 40, 74], [ 40, 73], [-70, 73], [-70, 74] ] ],
  ]);

geometry.transform('EPSG:4326', 'EPSG:3857');

var vectorLayer = new ol.layer.Vector({
    map: this.map,
    source: new ol.source.Vector({
        features: [new ol.Feature({
            geometry: geometry
        })]
    }),
    style: new ol.style.Style({
        fill: new ol.style.Fill({
            color: 'rgba(127, 64, 127, 0.4)'
        })
      })
});

var altitudeMode = 'clampToGround';
vectorLayer.set('altitudeMode', altitudeMode);

map.addLayer(vectorLayer);

var ol3d = new olcs.OLCesium({map: map, target: 'map3d'});
ol3d.setEnabled(true);
var scene = ol3d.getCesiumScene();
scene.globe.depthTestAgainstTerrain = true;