Learn CSS Transforms by Building a Penguin - Step 17

Tell us what’s happening:
Describe your issue in detail here.
Set the stack level of the mountain element such that it remains directly behind the .ground element
so, How can i do that? and when i wrote the following code , there is an error with this message " You should use the z-index property to change the stack level."

  **Your code so far**
<!-- file: index.html -->
<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8" />
  <link rel="stylesheet" href="./styles.css" />
  <title>Penguin</title>
  <meta name="viewport" content="width=device-width, initial-scale=1.0" />
</head>

<body>
  <div class="left-mountain"></div>
  <div class="penguin"></div>
  <div class="ground"></div>
</body>
</html>
/* file: styles.css */
body {
background: linear-gradient(45deg, rgb(118, 201, 255), rgb(247, 255, 222));
margin: 0;
padding: 0;
width: 100%;
height: 100vh;
overflow: clip;
}

.ground{
z-index:0deg;
}
.left-mountain {
width: 300px;
height: 300px;
background: linear-gradient(rgb(203, 241, 228), rgb(80, 183, 255));
position: absolute;
transform: skew(0deg, 44deg);
z-index:0deg;

}

.penguin {
width: 300px;
height: 300px;
margin: auto;
margin-top: 75px;
}

.ground {
width: 100vw;
height: 400px;
background: linear-gradient(90deg, rgb(88, 175, 236), rgb(182, 255, 255));
z-index: 3;
position: absolute;
}
  **Your browser information:**

User Agent is: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/89.0.4389.114 Safari/537.36 Edg/89.0.774.68

Challenge: Learn CSS Transforms by Building a Penguin - Step 17

Link to the challenge:

Your issue is here.
Hint: You might want to increase the z-index.
Deg is not necessary to use, you can just enter a number.