Thus, they are ideal for the acquisition of longer time series data. AUVs and stationary observatories are equipped with multiple sensors which, combined with their long endurance, can produce large amounts of data, especially when video and bathymetric data are captured. For instance, the multibeam surveys with AUVs are usually set to autonomous missions of 24 h, surveying a specific area. All the acquired data are retrieved and analysed once the mission is finished and the AUV is recovered [3]. Multibeam and side-scan sonars are frequently acquired simultaneously. The data volume is in the order 800 MB/day for multibeam data and 10 GB/day for side-scan data. The collected data need to be transferred to the surface to be processed and analyzed. When considering for instance deep-sea operation, surfacing frequently is unpractical due not only to the time and energy spent but also to the disturbance induced relative to the original mission. AUVs typically upload the data at the end of the mission, which causes delay in data processing and visualization and introduces significant dead-times between consecutive missions. This delay precludes possible adjustments in the AUV's mission (or other AUV's mission in a multiple-vehicle mission) due to the inability of onboard devices to process the collected data in real-time. Enabling broadband communications between the AUV and a central station so that the collected data can be timely uploaded along the mission is the solution for this problem. Current underwater communications solutions can only provide either long-range narrowband communications or short-range broadband communications. Acoustic communications are the most commonly used solution. However, despite the long-range capability, their high propagation delay and low bitrate render them unsuitable for timely  
Figure 1: EGIM observatory deployed in July 2017 and recovered in August 2018 at the Lucky Strike hydrothermal vent site [1; 2]. video transmission and transfer of high data volumes [5]. Optical communications, using LEDs or lasers, are able to increase the throughput to tens of Mbit/s. Despite the technological advancements, the practical underwater optical communications range is limited to tens of meters due to the water turbidity and the need of line-of-sight and proper beam alignment mechanisms. Radio Frequency (RF) communications offer the same broadband communications capabilities as optical communications, without the need of line-of-sight or beam alignment. However, RF signals suffer from strong attenuation underwater, limiting the practical use of broadband RF communications to a few meters. GROW is a pioneering solution that aims to overcome the limitations of current underwater communications technologies and provide long-range, broadband underwater wireless communications between a Survey Unit (SU)--e.g., deep sea lander and survey AUV--and a Central Station Unit (CSU) at the surface--e.g., buoy, vessel, and Autonomous Surface Vehicle [6]. The GROW concept is illustrated in Figure 2. At the core of the concept is a Delay Tolerant Network (DTN) [7; 8] composed of small and agile AUVs--data mules--equipped with (1) high bitrate wireless communications (e.g., RF and optical) for short-range data transfer and (2) long-range low bitrate acoustic communications for control purposes. The data mules, traveling back and forth between the SU and the CSU, create a virtual bidirectional communications link. The GROW solution has been tested in lab environment using an underwater testbed composed of one SU, one CSU, and two Data Mule Units (DMUs) [9; 10]. The experimental results obtained show that it outperforms current acoustic communications by achieving equivalent throughputs up to 150 times higher within the typical range of operation of the acoustic communications.