Generating a random List() of List(List(Doubles))

I'm trying to figure out how to generate a list of random doubles through the range -50 to 50, with the length of list being 20. (so 20 elements of random doubles ranging from -50 to 50).

I then want to create a fixed number (could be any number, we'll say 3 for now) of List[List[Double]] with that randomized double list. I read up on the Random doc but it is still very confusing to me. This is what I currently have:

 val length: Int = 20
 val doubles: List[Double] = List()
 val listOf: List[List[Double]] = List(List())
 val rand = new Random()

Essentially, let's say I do generate a list of 20 elements with random doubles ranging from -50 to 50. I then want to generate a random number of lists that include the randomized list of doubles. Ex:

val doubles: List[Double] = List(-29.3,46.8,-17.0,9.2,1.4) // in this case, doubles has a length of 5)
val listOf: List[List[Double]] = List(List(-29.3,46.8,-17.0,9.2,1.4),List(-5.0,3.4,31.5,29.0,-41.3)) // in this case, the inner lists have a length of 5, and the fixed number is
//2 because listOf has a length of 2

I am also looking to approach this problem with no mutability. How can I generate a random list of doubles with the above specs, and then generate a list of random lists?