## 1 Introduction  
More than the 70% of the Earth's surface is covered with water (almost all of that are seas and oceans). New technologies have brought us new ways to monitor and sense aquatic environments. Marine surveillance, river and sea pollution detection and monitoring, and oceanographic data collection are needed to explore, protect, and commercial exploit the aquatic environment. Manypotential applications exist in this environment such as fish and mussels grow observation [1,2], deep-sea archaeology, seismic and volcanic prediction, oil monitoring and so on. Sensor technology has matured enough to be used in any type of environment. As time goes on, the market offers smaller sensor devices, with lower power consumption and higher processing capacity. Moreover, the appearance of new physical sensors has increased the range of aquatic parameters for gathering data. Because of the huge amount of unexploited resources in the ocean environment, there is a need of new research lines in the field of sensors and sensor networks [3]. Furthermore, many sensor and sensor networks features must be taken into account in this type of environment: the need of large number of nodes, limited energy of the nodes, short-distance radio communications, large propagation latency, low bandwidth capacity and high error rate. All these constraints make underwater sensors and underwater sensor networks a special case of the regular ones. This special issue has tried to collect the recent advances on underwater sensors and underwater sensor networks. It has been mainly focused on measuring, monitoring, surveillance and controlling underwater environments. The topics of interest in the call for papers are shown in Table 1.  
This Special Issue has finally included 28 papers, which have been classified as shown in Figure 1. Five papers focused on the underwater sensor node (with the topics physical sensor, hardware and node measurements) have been accepted, nine papers focused on underwater communications (with the topics communication transceiver and layer 1-2 protocols) have been accepted, and 14 papers on sensor networks (with the topics position/location, architecture and protocol, and sensor network measurements) have been accepted. It has been split in this manner because the papers included in the underwater communication item have not tackled the problem from the network perspective, but on a single communication between two sensor nodes. \begin{table}
\begin{tabular}{|c|p{142.3 pt}|} \hline
**Topic** & \multicolumn{2}{c|}{**Suggested Subtopics**} \\ \hline \multirow{4}{*}{Sensor nodes} & Sensor nodes to measure water parameters (salinity, conductivity, turbidity, pH, oxygen, temperature, depth, _etc._) \\  & Sediments and pollution sensor nodes \\  & Acoustic sensors \\ \hline \multirow{2}{*}{Communications} & Modems for underwater sensor networks \\  & Optical, acoustic and electromagnetic communications for underwater sensor networks \\ \hline \multirow{4}{*}{Sensor networks} & Underwater sensor network architectures \\  & Wired and wireless protocols for underwater sensor networks \\  & Localization systems for underwater wireless sensor networks \\  & Target tracking using underwater sensor networks \\ \cline{1-1}  & Study cases, implementations and real deployments of underwater sensor networks \\ \cline{1-1}  & Routing protocols specialized for underwater wireless sensor networks \\ \cline{1-1}  & Cooperation in underwater sensor networks \\ \cline{1-1}  & Modeling and simulation in underwater sensor networks \\ \cline{1-1}  & Data collection, storage, and retrieval by underwater sensor network \\ \hline \end{tabular}
\end{table} 
Table 1: Classification of topics included in the Special Issue. To provide a better view of this classification when studying every item, we also present Figure 2, which splits this classification into levels.