Tectonic plates aren't just "deep in the ground" - they are the ground! They make up the Earth's lithosphere, which is the rigid outermost layer of the planet, including the crust and the uppermost part of the mantle.

Here's a breakdown:

* Crust: This is the thinnest and outermost layer, ranging from about 3-5 miles (5-8 kilometers) thick under the oceans to about 25 miles (40 kilometers) thick under continents.

* Upper Mantle: This is the layer beneath the crust, extending down to about 400 miles (660 kilometers).

* Lithosphere: This is the combined crust and upper mantle, forming the rigid, "rocky" plates that move on top of the softer layer below.

So, the depth of tectonic plates depends on whether you're talking about the oceanic crust (thinner) or continental crust (thicker). However, the lithosphere itself, which is the plate, is generally considered to extend to about 100 kilometers (62 miles) deep.

It's important to note that the boundary between the lithosphere and the softer layer below (the asthenosphere) isn't a sharp line but a transition zone.