The discovery of immense ridges and trenches stretching thousands of kilometers across the seafloor in the 1950s led to acceptance of the theory
of

Plate tectonic theory

Explanation:

Plate tectonic theory states that the earth contains numerous small and large lithospheric plates. These plates constantly moving from one place to another in three different ways such as

1) Convergent plate motion- here two plates move towards each other.

2) Divergent plate motion- here two plates move in the opposite direction, away from each other

3) Transform plate motion- here two plates slide passes each other.

This plate motion occurs due to the convection current that generates on the mantle because of the tremendous heat radiated from the core of the earth and this entire process is commonly known as the plate tectonic theory.

In the case of convergent plate motion, the denser plates get subducted beneath the lighter one and form a trench that appears to be a depression within the oceans and there forms a zone known as the subduction zone.

In the case of a divergent plate motion, it then forms a ridge, which later forms an ocean and the zone of separation of these two plates is known as the mid-oceanic ridge.

So a lot of ridges and subduction zones are present on earth.