.bubble {
    position: absolute;
    bottom: -200px;
    background-color: var(--red);
    border-radius: 50%;
    animation: rise 10s infinite;
}

@keyframes rise {
    0% {
        bottom: -200px;
        opacity: 0;
    }
    50% {
        opacity: 1;
    }
    100% {
        bottom: 100%;
        opacity: 0;
    }
}