SEOUL:  North Korea has fired a barrage of missiles, artillery pieces and rockets into the waters off its east coast, including a medium-range ballistic missile that fell near Japan's territorial waters this week.

The launches are meant to test its weapons systems, express anger in times of standoffs with South Korea and the United States, or prove it has the capability to attack its archrivals.

But little is known about what happens to those weapons. Do they cause any environmental problems in the ocean? Are some countries trying to retrieve the missile parts to study them?

Here is what we know about the likely fate of the weapons in the sea, which in some places is 3,000 meters (1.9 miles) deep or possibly deeper.

Where are the missiles now?

North Korea's Rodong missiles are 15 metres (49 feet) long and most other weapons it has fired are shorter. The parts are scattered in the deep ocean, and Professor Roh Taeseong at South Korea's Inha University says "it's like throwing grains of sand into Seoul's Han River."

North Korea often launches its missiles from different sites and fires them for different distances depending on what it wants from each launch. This means there is little chance of missiles landing in the same area. North Korean missiles are also known for poor accuracy, so it's highly unlikely that many pieces would end up near each other, even if North Korea is aiming at the same area.

When missiles hit the ocean, they face a huge impact that can break them into multiple pieces. "It's like hitting a concrete floor," said analyst Chae Yeon-seok at South Korea's state-run Korea Aerospace Research Institute. He said the Rodong missile fired Wednesday must have shattered into more than 10 pieces.

Some other experts say the amount of damage can depend on the angle of impact, with missiles entering at straighter angles suffering less damage.

Retrieving missile parts

In the case of a launch of a new or particularly threatening design, South Korea, the U.S. and Japan are likely to search for fragments and salvage what they can. Some parts from those rockets, particularly their first stages, can be fairly large. The three-stage Unha-3 rocket which North Korea used to send a satellite into space in 2012 was 30 meters (98 feet) long.

But it's virtually impossible to recover meaningful fragments of smaller missiles once they sink into the sea.

Even if South Korea or other countries were to successfully recover parts of a Rodong missile, it's unlikely they would learn anything new about the Soviet Scud-based weapon they have already analyzed for decades.

After Wednesday's launch, Japan still dispatched destroyers and P-3C Orion surveillance aircraft to search for any debris. Roh described the move as a "political show" by the conservative government of Japanese Prime Minister Shinzo Abe, which wants to build a stronger military by stressing to its people how dangerous North Korean missiles are.

Can ships be hit by north korean missiles?

It's likely that North Korea uses dummy warheads when it test-launches missiles. They won't explode on impact, and the missiles are statistically highly unlikely to hit a ship, so that danger is negligible.

North Korea has announced danger zones in accordance with international rules when it launched rockets to place satellites into orbit, butgenerally doesn't do so when it fires missiles.

Washington, Seoul and Tokyo call the satellite launches a prohibited test of long-range missile technology.

Can missiles cause enviromental harm?

Probably not. Most North Korean missiles use toxic liquid propellants, but experts in South Korea say most recent missiles appeared to have crashed into the ocean after burning all their fuel. The huge ocean would dilute any remaining fuel.

North Korean state media said some recent missiles contained trigger devices for warheads to practice possible atomic attacks on South Korea. But outside analysts say they likely used dummy warheads without any plutonium or uranium.

How does North Korea assess its launches?

It's not known how North Korea monitors and tracks its missiles. Analysts in South Korea say it must use radars to determine whether missiles land in targeted areas.

The best way would be to send ships near the impact zone to see the results of launches, but it's unknown if North Korea does so.

Why do North Korean missile tests matter?

Any country with a military conducts weapons tests. North Korea's tests make news because it is openly developing nuclear weapons and wants to place them on missiles capable of reaching faraway targets such as the mainland United States.

Under young leader Kim Jong Un, who took power in late 2011, North Korea has fired weapons more frequently than in the past.

It previously often deployed new missiles without flight-testing them to save costs. It may have felt that flight-tests weren't necessary when its missiles were slight redesigns of already-proven Chinese and Russian models, according to Kim Seung Jo, a professor at Seoul National University.

Earlier this year, North Korea suffered a series of failures before sending a new intermediate-range Musudan missile more than 1,400 kilometers (870 miles) high. It also fell into the ocean.