Royal Marines from 43 Commando have been sharpening their close-quarter combat skills in Scotland.
The 550-strong unit, which leads the security of the UK's nuclear submarine fleet carrying the Trident missile system, has had to move its training due to coronavirus lockdown restrictions.
Personnel would usually be overseas, but have instead been at the Police Scotland Training College in Jackton.
The site is where the police's armed officers hone their skills.
During training, their task is to work in teams, clearing rooms one-by-one, building their capability of operating in urban and industrial environments.
Each door they open could potentially expose the Royal Marines to risk, building their focus and reactions.
"Surprise and momentum are good, and lots of aggression as well," explained Marine Cullen, 43 Commando at the facility used to sharpen the skills of armed police officers.
"We'll never go into a room by ourselves so you're relying on your oppo next to you to cover your back.
"Moving into those rooms where there is a threat, we don't necessarily know what's in there or where he's positioned - we're relying on the tactics we train here.
"Also, relying on the bloke next to you to protect your back."
Tackling unknown threats up close, the unit has adapted its training while the UK continues to fight a well-known yet invisible enemy.
"This time of year normally we'd be overseas, either in the Netherlands or over in Gibraltar, but because of COVID we're restricted," said Marine Cullen."
"Training at Jackton's given us just somewhere new to train, a different facility - something we're not used to.
"Instead of having an idea of what might be around the corner or what we're coming up against, it's completely new.
"We're thinking on our feet and using our initiative."
While the less familiar environment in Jackton's police facility keeps 43 Commando on its toes, the Royal Marines are able to make the most of Remotely Piloted Aerial System (RPAS) drone technology and a miniature 'throwbot' to see what waits on the other side of urban walls.
"What that gives is the marine and the tactical command on the ground the extra bit of initiative to make decisions quicker, and ultimately make those decisions better informed," Lieutenant Roberts, 43 Commando said.
"It's all about making the marine as operationally effective as he can be."
Keen to seen the marines adapt alongside the wider UK to protect its nuclear capability, Lt Roberts added: "COVID-19 has provided a degree of friction to all organisations nationwide and the military certainly has not been exempt from that.
"The continuous at-sea deterrence which we support - that remains continuous.
"If those operations continue our mission continues and, therefore, training continues."