For most people, dangling over the edge of the tallest man-made structure in the U.S. is a terrifying prospect. But this is what several daring workers were doing today as they took stain samples from the 'stainless' steel surface of the Gateway Arch in St Louis. The first stage of the sampling, costing $340,000 in total, saw men rappel down the 630ft monument's west face for the first time in its 49-year history. They removed rust-colored stains from its surface, before sending them to the National Park Service for testing in a bid to figure out what is causing them. Scroll down for video . Daring: A construction worker dangles from a rope as he takes stain samples from the 'stainless' steel surface of Gateway Arch in St Louis . Iconic monument: The first stage of the sampling, costing $340,000 in total, saw men rappel down the 630ft monument's west face for the first time in its 49-year history. Above, the man was dangling near the top of the Arch, which is the tallest man-made structure in the U.S . In the past, officials have struggled to find the source of the mysterious stains, with engineers across the U.S. agreeing the Arch should be cleaned soon. This has been deemed particularly important as the iconic Missouri monument has not been cleaned since it was first topped off in 1965. So, to obtain samples of steel to test, construction employees were forced to dangle over the side of the famous Arch from ropes, STL Today reported. They were then required to use suction cups to hold themselves in place while they took samples from the west side of the monument's north leg. At work: The workers (including this one, pictured) removed rust-colored stains from its surface, before sending them to the National Park Service for testing in a bid to figure out what is causing them. In the past, officials have struggled to find the source of the mysterious stains . Well-secured: To obtain samples of steel, workers were forced to dangle over the side of the famous Arch from ropes. They were then required to use suction cups (pictured) to hold themselves in place while they took samples from the west side of the monument's north leg . Workers largely obtained stained steel from the Arch's underside, which is where most of the dirt and visible stains appear. And they were successful in their efforts - managing to complete the sampling by noon. Officials expect to receive a report on the stains and how best to prevent them from developing in January. However, the work has not always been plain sailing - construction companies have been forced to delay the project several times due to  weather. In the past week, strong winds and heat from the sun beating down on the Arch's metal sides have led to the sampling being put back. Staggering: Workers largely obtained stained steel from the Arch's underside, which is where most of the dirt and visible stains appear. And they were successful in their efforts, completing the sampling by noon. Above, workers collect samples from the Arch's exterior . Iconic: The Gateway Arch (pictured), situated in St Louis in the state of Missouri, has not been cleaned since it was first topped off in 1965 . This afternoon, Ann Honious, public information officer for the Jefferson National Expansion Memorial, comprising of the Gateway Arch, the Museum of Westward Expansion, and the Old Courthouse, said: 'Despite the delays, it was very successful. This was a difficult project that took years to plan.' Meghan Schrumpf, 40, who was watching the workers dangle from the Arch, said she was delighted to see the structure was being maintained. 'This is what we are known for,' said the account manager from  St Louis County. 'This is a big monument, we need to do what we can to keep it up.' The sampling was performed by Chicago-based contractor Wiss, Janney, Elstner Associates. Don't look down! The sampling was performed by Chicago-based contractor Wiss, Janney, Elstner Associates. Above, workers on the Arch .
Daring construction workers were spotted dangling over the edge of 630-foot Gateway Arch in St Louis today .
They removed rust-colored stains from its surface, before sending them to National Park Service for testing .
First stage of sampling, costing $340,000, was first time workers have cleaned the Arch in its 49-year history .
Workers successfully completed the sampling by noon; officials expect to receive official report in January .
However work has not always been plain sailing - the project has been delayed several times due to weather .
For most people, dangling over the edge of the tallest man-made structure in the U.S. is a terrifying prospect. But this is what several daring workers were doing today as they took stain samples from the 'stainless' steel surface of the Gateway Arch in St Louis. The first stage of the sampling, costing $340,000 in total, saw men rappel down the 630ft monument's west face for the first time in its 49-year history. They removed rust-colored stains from its surface, before sending them to the National Park Service for testing in a bid to figure out what is causing them. Scroll down for video . Daring: A construction worker dangles from a rope as he takes stain samples from the 'stainless' steel surface of Gateway Arch in St Louis . Iconic monument: The first stage of the sampling, costing $340,000 in total, saw men rappel down the 630ft monument's west face for the first time in its 49-year history. Above, the man was dangling near the top of the Arch, which is the tallest man-made structure in the U.S . In the past, officials have struggled to find the source of the mysterious stains, with engineers across the U.S. agreeing the Arch should be cleaned soon. This has been deemed particularly important as the iconic Missouri monument has not been cleaned since it was first topped off in 1965. So, to obtain samples of steel to test, construction employees were forced to dangle over the side of the famous Arch from ropes, STL Today reported. They were then required to use suction cups to hold themselves in place while they took samples from the west side of the monument's north leg. At work: The workers (including this one, pictured) removed rust-colored stains from its surface, before sending them to the National Park Service for testing in a bid to figure out what is causing them. In the past, officials have struggled to find the source of the mysterious stains . Well-secured: To obtain samples of steel, workers were forced to dangle over the side of the famous Arch from ropes. They were then required to use suction cups (pictured) to hold themselves in place while they took samples from the west side of the monument's north leg . Workers largely obtained stained steel from the Arch's underside, which is where most of the dirt and visible stains appear. And they were successful in their efforts - managing to complete the sampling by noon. Officials expect to receive a report on the stains and how best to prevent them from developing in January. However, the work has not always been plain sailing - construction companies have been forced to delay the project several times due to  weather. In the past week, strong winds and heat from the sun beating down on the Arch's metal sides have led to the sampling being put back. Staggering: Workers largely obtained stained steel from the Arch's underside, which is where most of the dirt and visible stains appear. And they were successful in their efforts, completing the sampling by noon. Above, workers collect samples from the Arch's exterior . Iconic: The Gateway Arch (pictured), situated in St Louis in the state of Missouri, has not been cleaned since it was first topped off in 1965 . This afternoon, Ann Honious, public information officer for the Jefferson National Expansion Memorial, comprising of the Gateway Arch, the Museum of Westward Expansion, and the Old Courthouse, said: 'Despite the delays, it was very successful. This was a difficult project that took years to plan.' Meghan Schrumpf, 40, who was watching the workers dangle from the Arch, said she was delighted to see the structure was being maintained. 'This is what we are known for,' said the account manager from  St Louis County. 'This is a big monument, we need to do what we can to keep it up.' The sampling was performed by Chicago-based contractor Wiss, Janney, Elstner Associates. Don't look down! The sampling was performed by Chicago-based contractor Wiss, Janney, Elstner Associates. Above, workers on the Arch .
For most people, dangling over the edge of the of the Gateway Arch in St Louis. The first stage of the sampling, costing $340,000 in total, saw for the first time in its 49-year history. They removed rust-colored stains from its surface, before sending them to the National Park Service monument: The first stage of the sampling, its surface, before sending them to the National complete the sampling by noon. Officials expect prevent them from developing in January. However, the work has not always been plain sailing - the project several times due to  weather. In the