## 2 Related Work  
Initially, the concept of digital underground mines was introduced with a very narrow vision of limited communication and the involvement of an operator. Now, this concept can be divided into separable categories of wireless communication and expert systems for safety and rescue in underground mines, personal position monitoring, and database systems. A wide range of literature on the present wireless communication and environmental monitoring in underground coal mines is summarized in [11]. WSNs use in underground mines is a new area of research [12]. Recently, a highly robust smart system was implemented at Shangwan Coal Mine, Erdos, China [13]. They upgraded a previously developed cable environmental monitoring system to a WSN. This system includes environmental monitoring and is capable of periodic inspection and interruption. An algorithm for early fire detection that includes alarms was introduced as a WSN in a bord-and-pillar mine [14; 15]. The most promising feature of this system is the early fire detection and the ability to locate a fire in the underground mine. Many existing studies [16; 17] have implemented WSNs with focus on low power and low operational costs in real underground conditions to enhance safety and productivity. From the perspective of wireless routing, a minimum delay maximum lifetime (MDML) [18] is an energy efficient, cost-effective, and reliable routing algorithm which has been specifically designed for the emergency and non-emergency transmission in underground coal mines. [PERSON] et al. [19] proposed a virtual multiple-input multiple-output (V-MIMO), combined with a orthogonal frequency division multiplexing to solve challenges of diffraction, attenuation, and scattering in WSNs of underground mines. The simulation results showed an enhanced wireless transmission reliability and reduced bit error rate as compared to simple single-input and single-output (SISO) in underground mines. [PERSON] and [PERSON] [20] developed a Structure-Aware Self-Adaptive (SASA) WSN based on stationary mesh nodes to locate miners. This system utilizes a normal beacon strategy for the detection of falling roofs, along with a limited monitoring of environmental parameters in underground coal-mines. WSNs with various indoor localization algorithms such as RSS, time-of-arrival (ToA), time difference of arrival (TDoA) [21], and angle of arrival (AoA) [22] have been widely implemented. Recently, RSS-based indoor localization has been widely adopted because of its simple calculations and the fact that it effectively estimates the mobile node (MN) position without requiring any additional hardware [23]. [PERSON] et al. [24] proposed an RSSI approach for indoor localization using Bluetooth nodes with a reduced accuracy error. [PERSON] et al. [25] presented a sensing platform based on the Wasmote and Meshlium gateway. This platform enabled the enhanced wireless (ZigBee/802.15.4) communication between the sensing platform and routing nodes (RNs) with the implementation of the data-forwarding approach. In addition to all of this, [PERSON] et al. [26] implemented cloud services for mine informatics and information management. In brief, miner's localization, tracking, gas monitoring, and event identification based on online cloud services can be effectively integrated into a single platform to further enhance underground coal mine safety. However, previous works related to mine safety are limited to basic monitoring and some event reporting. They lack the extensive and comprehensive system level integration of a wide range of safety aspects that includes information related to personnel and the underground mine itself. They are not sufficient enough to ensure comprehensive safety of the dynamic underground environment of the mine. Therefore, the solution must involve the seamless integration at local and global levels, and it must be scalable for large underground mines. Furthermore, the system must be cost-effective and should be efficient enough to identify abnormal events.