Tunnel

<div id="a">
  <div id="b">
    <div id="c"></div>
  </div>
</div>

<style>
  body {
    background: #6592cf;
  }

  body,
  div {
    display: grid;
    place-content: center;
  }

  body > div {
    background: #243d83;
    height: 250px;
    width: 250px;
  }

  body > div > div {
    height: 150px;
    width: 150px;
    background: #6592cf;
    rotate: 15deg;
  }

  div > div > div {
    width: 75px;
    height: 75px;
    background: #243d83;
    rotate: 15deg;
  }
</style>