First we have to understand what is tranches: In English dictionary trench meaning a portion or a slice or a part.
Here the trench meaning a famous long, narrow topographic depression of the ocean floor. It's generally parallel and about 200 km from volcanic arc(it is a chain of volcanoes forms above a subduing plate, position in an arc shape)

HISTORY OF term "TRENCH":

A trench is a type of excavation or depression in the ground that is generally deeper than it is wide (as opposed to a wider gully, or ditch), and narrow compared with its length (as opposed to a simple hole or pit).

In geology, trenches result from erosion by rivers or by geological movement of tectonic plates. In civil engineering, trenches are often created to install underground utilities such as gas, water, power and communication lines. In construction, trenches are dug for foundations of buildings, retaining walls and dams, and for cut-and-cover construction of tunnels. In archaeology, the "trench method" is used for searching and excavating ancient ruins or to dig into strata of sediment material. In geotechnical engineering, trenches serve for locating faults and investigating deep soil properties.

 

OCEANIC TRENCH:

Globally, there are over 50 major ocean trenches in the world which cover 1.9 million  are km² and about 0.5% of oceans.

 

HOW OCEAN TRENCH ARE FORM:

When the leading edge of dense tectonic plate and leading edge of a less-dense plate meet together,
 the dense plate bends downward then the place where the plate take out, that place is called subduction zone. 
The subduction of plate carrying continental crust  and plate carrying oceanic crust  as the result some oceanic trenches are formed. 
Continental crust is much  lighter than oceanic crust and that's why oceanic crust will always subduct.
Rarely, ocean tranches can be formed where two plate carrying oceanic crust.

TOP 5 DEEPEST TRANCHES IN THE WORLD:

1) MARIANA TRENCH:-

It's known as the deepest part or portion of the earth surface. It locates in the western Pacific Ocean about 200 kilometers east of Mariana islands; 
it has about 2550 km in length and 69 km in width.
If Mount Everest were dropped in to the Mariana trenches; its peak would still be more than a mile(1.6 kilometers) underwater.
It is the part of a global network  of deep thought that cut across the ocean floor. 

HISTORY OF MARIANA TRENCH :

In 1875 the British ship H.M.S Challenger(As the part of the first global oceanographic cruise) were first plumbed the depth of 4,475 fathoms(about 5 mile or eight kilometers) using a weighted sounding rope in 1951, the British vessel H.M.S Challenger II returns to the sport with an echo-sounder and measure a depth of nearly 7 miles(11 kilometers).
The majority of the Mariana trench is now a U.S. Procter zone as a part of the Mariana trench.  The Mariana National monument.  It established by president George W. Brush in 2009.

HOW THE MARIANA TRENCH FORMED:

When massive slabs of oceanic crust collide in a subduction zone, then the Mariana trench was created.
One piece of oceanic crust push and pulled under near the other at the subduction zone, sinking in to the earth, the layer under the crust where the two piece of crust intersect, a deep trench formed above the bond in the sinking crust.
In this case Pacific Ocean crust is bending below the Philippine crust.

LIFE IN MARIANA TRENCHES:

The depth of the Mariana trench makes it one of the deadline place on the planet it's forever covered in darkness, water temperature is below 0 degree Celsius. 
It impossible  for life as we know it to exist is the extreme water pressure and tonnes per square inch increases with depth. That place human body would collapse in the blink of an eye under this pressure, lungs would filled with air and bones would crust.

2)TONGA TRENCH:

 More than 3,000 miles(About 5,000 kilometers) east of Australia in the South Pacific Ocean, lies a group of 176 beautiful islands that comprise the kingdom of Tonga.
Tonga trench is the second-deepest trench in the world. It is the deepest trench in the Southern Hemisphere. Tonga trench has average depth of 20,000 feet(6,000 m) and the width of about 50 miles(80 km), it reaches maximum depth of 35,702feet(10,882 m).

HOW TONGA TRENCH FORM:

Like other deep ocean trench, the Tonga began to form millions of year ago in  a process called subduction ; in this process the two tectonic plates grind together, forcing one under others. In this case the edge of  the pacific plate was forced under the Tonga plate, a process still going on today. In fact, the pacific plate being subducted at about 9.4 inches a year. That may not sound like much, but it faster than any other tectonic plate movement on the planet.

3)PHILIPPINE TRENCH:

The third-deepest point  in the world. The average depth of Philippine trench is 10.54 kilometers below sea level. It locates the Philippine Sea, spreads in a length  of 1,320 km and 30 km width  in the east of the Philippines.

 

HISTORY OF PHILIPPINE TRENCH:

It's said that scientists consider the Philippine trench as the planet's deepest point until 1970. According to scientists, the Philippine trench is younger than 8-9 million year ago. 

HOW PHILIPPINE TRENCH FORM:

The Philippine trench formed from a collision between the Palawana and Zamboanga plates. This caused a change in geological processes, going from a convergent zone to a subduction zone. The subduction zone is located west to east of the Philippine Islands.

4)KURIL-KAMCHATKA TRENCH: 

Kuril- Kamchatka Trench is another fourth-deepest part of ocean belonging to the Pacific Ocean, extending for about 1,800 miles (2,900 km) north-south, it has a maximum depth of 34,587 feet (10,542 meters) and covers a total area of 102,000 square miles (264,000 square km). This trench lies at a considerable depth of 10.5 km below sea level. Lying close to Kuril Island and off the coast of Kamchatka, this trench is responsible for a number of oceans bed volcanic activities in the region.

HOW KURIL-KAMCHATKA TRENCH FORM:

The trench was formed due to the subduction zone that was developed in the late Cretaceous, which created the Kuril island and the Kamchatka volcanic arcs.

5)KERMADEC TRENCH: 

Kermadec Trench ,submarine trench in the floor of the South Pacific Ocean, about 750 mi (1,200 km) in length, forming the eastern boundary of the Kermadec Ridge. The two together comprise the southern half of the Tonga–Kermadec Arc, a structural feature completed to the north by the Tonga Trench and Ridge. The Kermadec Trench reaches a maximum depth of 32,962 ft (10,047 m).
Formed by the subduction of the Pacific Plate under the Australian Plate, the trench exhibits steep adjacent slopes, that, north of 32°S on the under riding plate, can exceed 40 degrees due to an extensive network of fault escarpments up to 1000 m in height and that are laterally extensive over many tens of kilometers. General oceanographic conditions and the characteristics of the fault escarpments, the deep trench axis, and the north (Osborn Seamount) and south (Hikurangi Trough and Rapuhia Scarp intersection) boundaries of the trench habitat are discussed. There is evidence of mass flow deposits, petite spot volcanism, and sedimentary basins that form complex habitats for benthic fauna. Overall data for biological communities that can be statistically compared are limited, but patterns in the distribution of selected Hadar communities are described.