The Mariana Trench ….. I have been thinking about this and I think, worked out a way of explaining the answer to your question.
Our planet, the Earth, is an interesting planet. It has a solid surface (we call the crust) that surrounds a molten (liquid) inside, called the mantle. Like a boat on water the surface floats on the molten liquid interior. Look at the image below:
Many millions of years ago the planet was just a hot molten sphere. Slowly the crust began to form. It didn’t all form at the same time so it became a little like a cracked egg shell with each bit floating and moving on the molten mantle. These pieces were the tectonic plates. The image below shows you the present positions of the tectonic plates.
You might be able to see what tectonic plate you are living on.
These plates are constantly moving (very, very slowly). When they bump into each other they can cause lots of problems, like earthquakes. The Philippine Plate and the Pacific Plate bumped into each other many millions of years ago and this made the Philippine Plate sink below the bigger Pacific Plate …. and the result the Mariana Trench. See the diagram below.
Where is the Mariana Trench?
I hope this makes sense Sabine. Feel free to ask another question.