Resolved Lost the ability to create solid objects

I've been working a project for some time, and have just created a test project to test out some theories. When creating the basic project, my gameObject falls straight through my plane. I've done this many times before, and I'm sure there's a tickbox or setting I've failed to tick. Can you help me? Here are the steps I've gone through:

1. Create a new project (I'm using 2021.1.14f1)
2. Add a plane and click transform -> reset (confirmed that it comes with a mesh collider). Leave settings to default
3. Add a 3D Object -> Cube
4. Add a Rigidbody and set Mass to 2, and Collision Detection to continuous
5. Add a BoxCollider. Leave settings to default
6. Raise the Cube up above the plane (Y: 4)
7. Click play

The cube drops and falls through the plane. What did I miss?

I've read about adding other Game Objects and adding RigidBody to them, then fixing the Y position so that the plane doesn't fall away, but surely a plane with a mesh collider should stop the cube?

 

Silly me. I had the isTrigger property set!