Some 8,600 personnel, 50 surface ships, 36 aircraft, and two submarines from 18 countries are in the Baltic Sea this month for Baltic Operations.
The annual BaltOps exercise, led this year by the US Navy’s recently revived 2nd Fleet in its first major European engagement, allows partners to practice air defense, anti-subsurface warfare, amphibious operations, and mine warfare.
Mines are especially dangerous in confined, heavily trafficked waterways, like the Strait of Hormuz or the Baltic Sea.
Bordered by six NATO members, the Baltic is littered with World War I- and II-era mines, and Russia is believed to have the world’s largest arsenal of naval mines — as many as a quarter-million, by one estimate.
“The Baltic Sea is of vital strategic importance for the alliance,” said NATO spokesperson Oana Lungescu, who stressed that the exercise was not targeted at any country but noted the deterioration of European security since Russia’s 2014 annexation of Crimea.
BaltOps 2019’s Mine Warfare Task Group had sailors and experts, including more than 70 divers, from 11 countries manning more than 15 mine-countermeasures ships, 15 unmanned undersea vehicles, five drone ships, and airborne mine-countermeasures systems.
“There is a lot of value in this exercise as it supports not only our US capability, but our work with partner nations in the mine-warfare space,” said Navy Lt. Cmdr. Daniel Claytor, officer in charge of a detachment from Helicopter Sea Combat Squadron 28.
Below, you can see how the US and NATO train for a uniquely complicated, and uniquely dangerous, form of warfare.

Researchers from the US Office of Naval Research and German Naval Research aboard FSG Konsort, conduct MCM Experimentations using an Mk 18 Mod 2 unmanned underwater vehicles during BaltOps in June 2019.
(US Navy photo by Chief Mass Comm Specialist Shannon E. Renfroe)

Lt. Alex Burtness, left, a field support representative for Mk 18 Mod 2, prepares to lower a Mk 18 Mod 2 from the stern of FSG Kronsor during experimental Mine Countermeasure operations at BaltOps in June 2019.
(US Navy photo by Chief Mass Comm Specialist Shannon E. Renfroe)

A Mk 18 Mod 2 is submerged from the stern of FSG Kronsort during experimental Mine Countermeasure (MCM) operations at BaltOps in June 2019.
(US Navy photo by Chief Mass Comm Specialist Shannon E. Renfroe)
The Mk 18 Mod 2 UUVs being tested had two kinds of sensor packages: The Small Synthetic Aperture Minehunter and the Autonomous Topographic Large Area Survey forward-looking sonar.
Small Synthetic Aperture Minehunter systems work on a range of wavelengths, providing fine-grain imaging of the seafloor and of small man-made objects as well as peering into the seabed to provide imagery and analysis of buried objects.
The SSAM II module used at BaltOps “provides higher resolution and is intended to hunt bottom mines,” said Navy Lt. Matthew Stroup, public affairs officer for the BaltOps 2019 Mine Warfare Task Group.
SSAM II “has two modes; linear [synthetic aperture sonar] mode for rapid search and circular SAS, which provides very high-resolution images to enable” reacquiring and identification, Stroup added.
Autonomous Topographic Large Area Survey forward-looking sonar, known as ATLAS, has a wide search-area width that’s meant for volume mine-hunting, Stroup said. It can also be used to gather information, including mapping of clutter and large-object detection on the seafloor and to gauge ocean depth.

A Mk 18 Mod 2 unmanned underwater vehicle is submerged from FSG Kronsort during experimental Mine Countermeasures operations at BaltOps in June 2019.
(US Navy photo by Chief Mass Comm Specialist Shannon E. Renfroe)
Mine warfare is key to maintaining sea lines of communications, particularly in ports and landing areas, said US Navy Rear Adm. Scott Robertson.
Robertson is commander of the Naval Surface and Mine Warfighting Development Center and led the BaltOps 2019 Mine Warfare Task Group.

Lyle Valtron, a field support representative, operates an Mk 18 Mod 2 aboard FSG Kronsort during experimental Mine Countermeasure operations at BaltOps 2019.
(US Navy photo by Chief Mass Comm Specialist Shannon E. Renfroe)

A mine countermeasure ship attached to Standing NATO Mine Countermeasures Group One (SNMCMG1) transports divers in preparation for an investigative dive during a mine countermeasure exercise in support of Baltic Operations 2019.
(NATO/CPO Brian Djurslev)

Naval Aircrewman 1st Class Patrick Miller operates the common console, used for both Airborne Laser Mine Detection System (ALMDS) and the Airborne Mine Neutralization System (AMNS), aboard a MH-60S Sea Hawk helicopter, June 2019.
(US Navy photo by Chief Mass Comm Specialist Shannon E. Renfroe)

A close-up of the common console aboard a MH-60S Sea Hawk helicopter during BaltOps 2019.
(US Navy photo by Chief Mass Comm Specialist Shannon E. Renfroe)

The Airborne Laser Mine Detection System aboard an MH-60S Sea Hawk helicopter heading to the Baltic Sea during BaltOps 2019.
(US Navy photo by Chief Mass Comm Specialist Shannon E. Renfroe)

A team of Norwegian explosive ordnancemen prepare to detonate a World War II-era, air-laid mine weighing approximately 1,000 pounds in the Baltic Sea, June 2019. The team is operating as part of an 11-nation Mine Warfare Task Group in BALTOPS 2019.
(US Nav photo by Lt. Matthew A. Stroup)

A team of Norwegian explosive ordnancemen prepare to detonate a World War II-era, air-laid mine weighing approximately 1,000 pounds in the Baltic Sea, June 2019.
(US Navy photo by Chief Mass Comm Specialist Shannon E. Renfroe)

An unexploded A Mark I-VI mine at the bottom of the Baltic Sea. It was detected, identified, and detonated by Norwegian and Danish naval personnel during BaltOps 2019 in June 2019.
(Royal Danish Navy)

A diver attached to Standing NATO Mine Countermeasures Group One (SNMCMG1) prepares an investigative dive during a mine-countermeasure exercise at Baltic Operations 2019.
(NATO photo by CPO Brian Djurslev)

A roughly 1,000-pound WWII-era air-laid mine detonates in the Baltic Sea after being discovered by the BALTOPS 2019 Mine Warfare Task Group and being rigged for detonation by a team of Norwegian explosive ordancemen in June 2019.
(US Navy photo by Chief Mass Comm Specialist Shannon E. Renfroe)
“There is a good chance we will find more of these mines as the exercise continues, and it’s reassuring to know our international task group has the training and expertise necessary to safely dispose of them,” Robertson added.
This article originally appeared on Business Insider. Follow @BusinessInsider on Twitter.