var earth = new OpenLayers.Layer.XYZ(
    "Natural Earth",
    [
        "http://a.tiles.mapbox.com/v3/mapbox.natural-earth-hypso-bathy/${z}/${x}/${y}.png",
        "http://b.tiles.mapbox.com/v3/mapbox.natural-earth-hypso-bathy/${z}/${x}/${y}.png",
        "http://c.tiles.mapbox.com/v3/mapbox.natural-earth-hypso-bathy/${z}/${x}/${y}.png",
        "http://d.tiles.mapbox.com/v3/mapbox.natural-earth-hypso-bathy/${z}/${x}/${y}.png"
    ], {
        attribution: "Tiles &copy; <a href='http://mapbox.com/'>MapBox</a>",
        sphericalMercator: true,
        wrapDateLine: true,
        numZoomLevels: 5
    }
);

var map = new OpenLayers.Map({
    div: "map",
    layers: [earth],
    center: [0, 0],
    zoom: 1
});