Explain why/how are some of the deepest parts of the ocean near land or near islands, whereas some of the shallowest parts of the ocean are in the middle of the Atlantic.

The unevenness on the surface of the planet Earth is due to subduction,

subduction near the land and eruption of magma at the middle of the Atlantic ocean. In the middle of the Atlantic, ocean lava is coming from the deeper part of the earth and erupts on the surface. The place where lava erupts In Atlantic ocean is also known as a Mid-ocean ridge.

Due to the eruption, huge mountains of lava will form, after cooling of lava followed solidification. Because of the mountain formation, the relative sea depth decrease at the center. Now if we carefully analyze the situation we come to the conclusion that due to lava eruption at mid-ocean ridges (MOR) or at the center of the ocean a new crust is forming.

Now if the new crust is forming at MOR it means the older crust also need to be disposed off, since we have fix space on the surface to accommodate crust. Now to dispose older crust the subduction is also going near the land When one plate subducting beneath the other plate, space is created which we called a trench.

Trenches are a deeper part of the world. So far the deepest part of the earth is Marina Trench.