Malaysia Airlines Flight MH370 vanished mid-air after taking off from Kuala Lumpur on March 8, 2014. There were 227 passengers and 12 crew members on board the aircraft, which was on its way to Beijing, China. The fate of the missing airliner has been shrouded in mystery ever since its disappearance.

While the Boeing 777 is believed to have crashed in the southern Indian Ocean, international search efforts have so far failed to provide any clues as to what happened to the plane.

On January 29, Malaysia's Department of Civil Aviation (DCA) officially declared the flight was an accident, adding that all 239 people aboard the aircraft were presumed dead. The announcement is in accordance with international aviation rules that allow families of the passengers to obtain compensation. Malaysian officials said they had not ruled out foul play and that the recovery of the missing aircraft remained a priority.

Satellite data revealed that MH370 continued to fly for over six hours after its disappearance from the radar and all the available information indicate the aircraft "entered the sea close to a long but narrow arc of the southern Indian Ocean," according to the Australian Transport Safety Bureau (ATSB), which is leading the underwater search for MH370 in the southern Indian Ocean. The search area has been narrowed to a 60,000 square kilometer (23,000 square mile) zone - and investigators have so far scoured more than 40 percent of it.

Commissioner Dolan: 'We must be cautious because of the limited information we have on aircraft's possible location'

In an exclusive DW interview, ATSB Chief Commissioner Martin Dolan talks about how confident he is about finding the plane's wreckage and says the search efforts confront significant challenges ranging from a lack of information to the remoteness and the size of the search area.

DW: Almost one year after the disappearance of Flight MH370, how confident are you that you are looking in the right place and will find the plane's wreckage?

Martin Dolan: We are cautiously optimistic. We are optimistic because we are confident the search methods we are using will detect the aircraft debris field on the sea floor, but we must be cautious because of the limited information we have in relation to the aircraft's possible location on the sea floor. The current priority search area was defined by the reconstruction of possible aircraft flight paths using a ground breaking analysis of satellite data and aircraft performance information.

This work has been performed and peer-reviewed by the experts in the Search Strategy Working Group, and we have confidence in this analysis. However, the search is a tremendous challenge, and while we are looking in the most probable area, we cannot be certain - there are simply no guarantees.

How many people and vessels from how many counties are invested in finding the plane at moment?

There are currently four vessels conducting the underwater search - Fugro Equator, Fugro Discovery and Fugro Supporter - which are being jointly funded by the Australian and Malaysian governments, and GO Phoenix, which is funded completely by the Malaysian government.

Each of these vessels carries a marine crew as well as the search specialists on board, who are responsible for operating the search equipment and reviewing the sonar data. In total, there are around 35 crew members on each ship. These ships spend up to 47 days at sea at a time.

'There are currently four vessels conducting the underwater search'

While the crew members aboard the ships face the most challenging working conditions, there are also many, many others involved in the search effort.

To name just a few, there are the members of the Search Strategy Working Group from organizations around the world, the team in the Joint Agency Coordination Centre, the Operational Search team at the Australian Transport Safety Bureau, the land-based staff of Fugro Survey and Phoenix International and the many Malaysian and Chinese Government officials.

It is a huge undertaking with many aspects and for it to be conducted effectively requires the work of a great many people.

What sort of equipment is being used and how does it work?

Three of the vessels- GO Phoenix, Fugro Discovery and Fugro Equator - conduct the underwater search using towed vehicles ("towfish") equipped with side scan sonar, synthetic aperture sonar, multi-beam echo sounders and cameras.

The towfish are all rated to 6,000 meters, and are towed on cables of up to 10 kilometers in length for weeks on end without being retrieved. While the devices are effective at covering large swathes of territory at reasonable speed, the towing arrangement means that they cannot search certain types of seafloor terrain as easily.

There are portions of the search area - especially rugged terrain or chasms, for instance - which do not lend themselves to the effective use of a towfish. In order to ensure those areas are searched thoroughly; Fugro Supporter joined the search, arriving in the search area on January 29, 2015. The vessel was equipped with an autonomous underwater vehicle (AUV). The AUV is self-propelled and highly maneuverable and is deployed to search a pre-programmed area of the sea floor.

After each underwater mission, the AUV will ascend and be recovered by the ship in order for the gathered data to be downloaded and the AUV's batteries to be changed out with a spare charged set. The AUV's maneuverability allows for the difficult areas of seafloor to be searched effectively.

The ATSB is coordinating all activities and assets associated with the underwater search, including the Fugro vessels and search vessels proved by the Malaysian government.

'The AUV is self-propelled and highly maneuverable and is deployed to search a pre-programmed area of the sea floor'

What have been the biggest difficulties and challenges in finding the plane so far, especially given the remoteness of the area where you are searching for the plane?

This operation has included many significant challenges, on many different levels. The first, as I mentioned earlier, is the relative lack of information about the aircraft's position and flight path after radar contact was lost at the northern tip of Sumatra. In most aircraft accidents, the last known position of the aircraft is relatively close to where it crashed and debris from the aircraft is found in comparatively close proximity. In this case, we know that MH370 continued to fly for many hours after it flew beyond primary radar.

Another element that presented a challenge was the mobilization of the underwater search effort. To begin in a timely fashion, we needed to clearly define the technical search requirements with expert assistance and then identify and engage highly-specialized services to conduct the search.

It may not be as high profile as working aboard one of the ships, but there were staff who put in very long days, well into the night, to prepare and assess the search tenders, prepare operational plans and risk assessments and negotiate the contract to ensure that the search operation is as effective, efficient and safe as possible.

Now that the underwater search is underway, we continue to face challenges. Some are associated with the search area. The remoteness and the size of the search area mean that every aspect of the operation must be planned and undertaken meticulously, with the safety and wellbeing of the crew our greatest priority.

Foul weather can hamper the work and make it risky, and ensuring that the ships and equipment are operating at their very best requires great diligence. While gathering the sonar search data can be difficult, there is also the work of analyzing and interpreting the data, which is why we have employed sonar experts of the highest caliber to oversee the work.

I would like to add that one challenge we have not faced, however, is any faltering in our commitment to finding the aircraft. The people working on this have shown incredible dedication, often going above and beyond what I could have asked of them.

'The remoteness and the size of the search area mean that every aspect of the search must be undertaken meticulously'

At what depth do you expect to find the plane and in which condition?

The search area has considerable variation in seafloor topography and depth, with some areas as deep as 6,000 meters - this is one of the elements that have made the search such a challenge. As to the condition of the aircraft, it is impossible to say with certainty. We expect that the aircraft will have broken up significantly when it impacted the surface of the sea and also as it subsequently sank to the seafloor, and we are preparing for every eventuality.

You haven't yet found any indication of the plane, but what can you tell us about what you have found in that part of the ocean? Have you found any previously unknown features?

Prior to the search for MH370, the seafloor in the search area had never been mapped in detail. Before the sidescan sonar work to locate MH370 could begin, it was necessary to conduct a bathymetric survey to ensure that the search equipment could be operated safely.

The survey vessels used multibeam sonar to gather data about the seafloor. That data was analyzed by experts at Geoscience Australia, revealing many seabed features for the first time. Newly discovered sea floor features include seamounts (remnant submarine volcanoes), ridges (semi–parallel) up to 300 meters high, and depressions up to 1400 meters deep (compared to the surrounding seafloor depths).

It also revealed finer-scale seabed features that were not visible in the previous low-resolution, satellite-derived bathymetry data.

'The search area has considerable variation in seafloor topography and depth'

If you do find the plane, do you have a recovery plan for such depths in the ocean?

While no wreckage from the missing aircraft has yet been found, preparations are being made so a recovery operation can be mobilized quickly and effectively when needed.

Responses to the request for expressions of interest will allow the ATSB to assess which organizations are able to supply the equipment and expertise required for any recovery operation. Decisions in relation to any recovery operation will be made jointly by the Australian, Malaysian and Chinese governments.

It's been said that this is the most expensive plane search in the history of aviation. Who is footing the bill?

Twenty-six countries have been involved in the search to date with all countries covering their own costs. Australia committed 90 million AUD, of which 60 million AUD is for the underwater search.

When do you expect to conclude the search for the plane in this area and, if the jet is not found, who is to decide on whether or not to continue the investigation?

Any decision on the future of the search will be made by the governments of Australia, Malaysia and China.

Martin Dolan is Chief Commissioner of the Australian Transportation Safety Bureau (ATSB).

The interview was conducted by Gabriel Domínguez.