## Materials and Methods
### VRMT Implementation  
The VRMT was developed to be used via an immersive VR HMD and its hand controllers. Specifically, the HTC Vive Pro [33] kit and ecosystem were used in the study, together with Valve Index [34] controllers to achieve finer control over the VE and more realistic interactions with the virtual objects. The HMD features a display resolution of \(1400\times 1600\) pixels per eye, spanning a horizontal \(110^{\circ}\) field of view with a 90 Hz refresh rate. Its native positional tracking leverages the infrared lasers emitted by the so-called base stations (built upon Valve's Lighthouse technology) which, combined with the HMD's built-in sensors, enable a 6 DOF outside-in tracking over an area of up to \(10\text{ m}\times 10\text{ m}\). The user interacts with the VE using the tracked controllers and their physical buttons.  
The VRMT application was implemented using the Unity [35] game engine and the SteamVR framework. All the logic and algorithms were written in the C# programming language. The 3D assets used to create the VE (in particular, to recreate a high-fidelity replica of the pump) were modeled with Blender [36]. Appropriate care was taken also to reproduce the audio cues from the real pump and equipment in VR.  
All the phases of the HCP training were integrated in the VRMT to offer a realistic replica of the real counterpart. Moreover, the VRMT was endowed with the possibility to let the trainees choose whether to experience the entire training (Figure 2) or just a subset of phases and tasks. The available tasks were pump placement, the delimitation of the operational field using poles and tape, the placement of the protective sheet on the river bank, pump management and the use of the safety or recovery rope.  
When experiencing all the devised phases, the trainees had to choose a given role between taking care of the aspiration or the delivery chain (the other role was handled by the VRMT). Appropriate care was devoted to the simulation of tubes and hoses, implementing all the necessary steps to assemble the aspiration and delivery chains, particularly regarding the coupling of the chain elements.  
Since most of the tasks involved the collaboration of two different operators, Non-Player Characters (NPCs) were added to the VRMT to assist the trainees during the whole experience; e.g., carrying the tubes (Figure 2e), helping during the assembly operations (Figure 2f,h or warning the user in case of leakages. Furthermore, the NPCs assembled the chain that was not chosen by the trainees. All the NPCs were managed using a common event-driven approach through finite-state machines.  
With the aim of minimizing the discrepancies between the real and the simulated VE, accessory visual feedback (e.g., highlighting) was implemented to support the interaction with virtual objects, whereas voiceovers were added to NPCs to describe the whole procedure and give a general context to the trainees.  
The implementation was validated by two officers of the Piedmont Region Civil Protection and Forest Fire Fighting Units. Overall, the validation had a positive outcome, and the officers highlighted how the experience was more \"real\" and engaging than initially conceived. It was also emphasized that the VRMT seemed truly capable of accurately representing the most practical aspects of the HCP procedure.  
Figure 2: Excerpt of few key moments of the arranged training procedure in the VRMT. In order: arrival at the deluged area (**a**), an operator wears the PPE (**b**), delimitation of the operational field (**c**), final operational field (**d**), two operators transporting a semi-rigid tube (**e**), assembly of tubes using hooks and levers (**f**), assembly of the delivery chain (**g**), assembly of two foldable hoes (**h**), overview of the two chains (**i**), immersion of the filter (**j**), checking of the filter and impeller valves (**k**), discharging of water in the delivery area (**I**).