How are major undersea cables laid in the ocean?

Grab some coffee or some snacks. Long answer!

Laying of cables in the oceans of our world is a fascinating business. Real men and women toil long and tedious hours to make this possible.

Submarine cables are laid down by using specially modified ships (sometimes even purpose built ships) that carry the submarine cable on board and slowly lay it out on the seabed as per the charts/plans given by the cable operator. The ships can carry with them up to 2,000 kilometers length of cable.
Depending on the equipment on-board the cable-ship, the type of plough used, the sea conditions and the ocean-bed where the cable is being laid-down, cable ships can do anywhere from 100-150km of cable laying per day. Newer ships and ploughs now do about 200 km of cable laying per day.

The ships are commonly referred to as cable-layers or cable-ships.

The cables are specially constructed for submarine operations as they have to endure harsh conditions as well as pressure.
Polyethylene
"Mylar" tape
Stranded metal (steel) wires
Aluminum water barrier
Polycarbonate
Copper or aluminum tube
Petroleum jelly
Optical fibers

These fibre optic cables carry DWDM

laser signals ( TCP/IP packets etc. ) at a rate of terabytes per second. They use optical repeaters to strengthen the signal which attenuates over long distances. These are powered by copper cables shown above.

They have a decade lifespan and costs vary (depending on the length of the cable). Typical costs for projects are anywhere from US$ 100 Million to $500 Million.

You might also want to read Do private telecommunications companies own the undersea cables that connect the internet across continents? How do they make money on the undersea cables? Who pays for them?

We don't use satellites because they can't carry terabytes of data for less than a billion dollars per communication line.