Nested flowchart/diagrams

I am creating a complicated and very large diagram, and I don’t want all the details to be visible at first glance. That’s why I need nested diagrams. I would like to enclose child diagrams inside a block that appears on the top‑level diagram. The child diagram should be visible only when I double‑click the block to enter it. Two layers of nesting would be fine, but if possible, having three or four levels would be excellent.