Come da titolo i sottomenù del menù appena creato vengono coperti dall'header e non riesco ad individuare la causa di ciò.

codice HTML:
<body>
    <nav>
        <ul>
            <li><a href="index.html">HOME</a></li>
            <li><a href="#">REGIONI</a>
                <ul>
                    <li><a href="#">NORD</a>
                        <ul>
                            <li><a href="regioni/lombardia/lombardia.html">LOMBARDIA</a></li>
                        </ul>
                    </li>
                    <li><a href="#">CENTRO</a>
                        <ul>
                            <li><a href="regioni/abruzzo/abruzzo.html">ABRUZZO</a></li>
                        </ul>
                    </li>
                    <li><a href="#">SUD</a>
                        <ul>
                            <li><a href="regioni/sicilia/sicilia.html">SICILIA</a></li>
                        </ul>
                    </li>
                </ul>
            </li>
        </ul>
    </nav>
    
    <header>
        <h1>Italia & Dintorni</h1>
    </header>

codice:
nav {
    margin: 100px 0;
    background-color: #000000;
}

nav ul {
    padding: 0;
    margin: 0;
    position: relative;
    list-style-type: none;
}

nav ul li {
    margin: 0 -7px 0 0;
    display: inline-block;
    background-color: #000000;
}

nav a {
    display: block;
    padding: 0 10px;
    color: #FFFFFF;
    font-size: 20px;
    line-height: 60px;
    text-decoration: none;
}

nav a:hover {
    background-color: lightgray;
}

nav ul ul {
    display: none;
    position: absolute;
    top: 100%;
}

nav ul li:hover > ul {
    display: inherit;
}

nav ul ul li {
    min-width: 170px;
    display: list-item;
    position: relative;
}

nav ul ul ul {
    position: absolute;
    top: 0;
    left: 100%;
}

header {
    position: relative;
    height: 400px;
    background-image: url(../images/header.jpg);
    background-repeat: no-repeat;
    background-size: 100% 100%;
    box-shadow: 2px 2px 5px gray;
}

h1 {
    font-size: 200px;
    font-weight: bold;
    color: gold;
    text-shadow: 1px 1px 10px gray;
    text-align: center;
    padding-top: 55px;
}

Un aiutino?