This paper presents a new class of ultra-thin soft robots (TS-Robots) with a thickness of 1.7 mm, capable of performing multimodal locomotion in both solid and liquid domains, including the ability to transition between them. The TS-Robots are designed with a Thin Soft Dielectric Elastomer Actuator (TS-DEA) and multiple Electrostatic Adhesive Pads (EA-Pads), enabling them to crawl, climb, swim, and transition between domains in narrow spaces. The TS-DEA uses a dual-actuation sandwich structure with a tunable Poisson's ratio tensioning mechanism, allowing for two types of gaits: linear and undulating. The TS-Robots can generate a remarkable output force (~41 times their weight) and speed (1.16 times Body Length/s and 13.06 times Body Thickness/s). The TS-Robots are capable of navigating through narrow gaps and other obstacles commonly found in built environments, such as those under doors or heavy objects, duct systems, and gaps within complex machinery. They can also collaborate with other robots, such as drones, to perform cross-domain locomotion. The TS-Robots have been tested in various environments, including the inspection of electrical generators for aero-engines, demonstrating their ability to access extremely narrow gaps in high-value industrial systems. The TS-Robots have been characterized for their static and dynamic properties, showing high force/weight ratios and efficient energy conversion. They have also been tested for locomotion in different domains, including solid and liquid, and have demonstrated the ability to perform crawling, climbing, steering, swimming, and landing. The TS-Robots can also be joined together to perform complex tasks, such as transitioning between surfaces and operating as manipulators. The TS-Robots have been shown to be effective in a variety of applications, including inspection of confined spaces within industrial plants and civil engineering buildings, performing security surveys of confined spaces, and accessing and observing hazardous environments. The design of the TS-Robots allows for the tuning of the resonant frequency of the TS-DEA by mechanically adjusting the stiffness of the tensioning mechanism, which is a fundamentally new method for improving DEA performance. The TS-Robots are more suitable for exploring complex-built environments, especially when needing to access near-millimetre and submillimetre-high narrow spaces, such as the gaps between components in large machines or industrial plants.This paper presents a new class of ultra-thin soft robots (TS-Robots) with a thickness of 1.7 mm, capable of performing multimodal locomotion in both solid and liquid domains, including the ability to transition between them. The TS-Robots are designed with a Thin Soft Dielectric Elastomer Actuator (TS-DEA) and multiple Electrostatic Adhesive Pads (EA-Pads), enabling them to crawl, climb, swim, and transition between domains in narrow spaces. The TS-DEA uses a dual-actuation sandwich structure with a tunable Poisson's ratio tensioning mechanism, allowing for two types of gaits: linear and undulating. The TS-Robots can generate a remarkable output force (~41 times their weight) and speed (1.16 times Body Length/s and 13.06 times Body Thickness/s). The TS-Robots are capable of navigating through narrow gaps and other obstacles commonly found in built environments, such as those under doors or heavy objects, duct systems, and gaps within complex machinery. They can also collaborate with other robots, such as drones, to perform cross-domain locomotion. The TS-Robots have been tested in various environments, including the inspection of electrical generators for aero-engines, demonstrating their ability to access extremely narrow gaps in high-value industrial systems. The TS-Robots have been characterized for their static and dynamic properties, showing high force/weight ratios and efficient energy conversion. They have also been tested for locomotion in different domains, including solid and liquid, and have demonstrated the ability to perform crawling, climbing, steering, swimming, and landing. The TS-Robots can also be joined together to perform complex tasks, such as transitioning between surfaces and operating as manipulators. The TS-Robots have been shown to be effective in a variety of applications, including inspection of confined spaces within industrial plants and civil engineering buildings, performing security surveys of confined spaces, and accessing and observing hazardous environments. The design of the TS-Robots allows for the tuning of the resonant frequency of the TS-DEA by mechanically adjusting the stiffness of the tensioning mechanism, which is a fundamentally new method for improving DEA performance. The TS-Robots are more suitable for exploring complex-built environments, especially when needing to access near-millimetre and submillimetre-high narrow spaces, such as the gaps between components in large machines or industrial plants.