This looks like something hard and smooth smashed it.

看起来像被硬而且平滑的东西打碎的。

Its walls were hard and smooth to the touch; it felt like stone, but it was not stone.

它的墙壁摸起来又硬又光滑。 它感觉像石头， 但它不是石头。

The walls were hard and smooth, of that stone unlike stone which we had seen in our tunnel.

墙壁坚硬而光滑，与我们在隧道中看到的石头不同。

We could wait a few days until the snow on the ground is much more packed down, so it'll be hard and smooth.

我们可以等几天，直到地面上的雪压得更厚，这样才会又硬又光滑。