Lets talk about making robots that can go under water or fly or go under ground.
Often roboticists got their start playing with RC airplanes or boats so if you haven’t started in that direction then it could be helpful to buy a few remote control cars or trucks and of course a flying drone.
Look at this amazing video of Eric Dirgahayu as he makes robots out of bending round PVC pipe and inserting microcontrollers and sensors and more! Here is his moving underwater fish (after he silicon water proofed his mini servo motor) https://www.youtube.com/watch?v=T08bQT7fyaI
You should spend some time observing nature or youtube to see how various creatures move.
Look on the ground, in the sky and underwater.
A dragonfly’s opposite legs move together.
Here is an amazing Youtube video by the Festo Robotics company as they show their latest insect creations and their human like arm movements. https://www.youtube.com/watch?v=ooqiY7hFWE8
Look at how the legs of a dog or cat or horse move.
Study a grasshopper or a mosquito or a lobster or a snake.
Watch how a fish moves and how a bird flies.
There are some amazing Youtube videos of remote control birds that actually look like real birds flying with the flapping wings. James Bruton has some new videos in September 2021 where he is trying to build a bird robot.
BirdKit.com sells a great looking wing flapping kit that allows you to build what is called an “ornithopter” which is basically something that looks like a bird flying with a 2 foot wingspan.
Of course you could do some Youtube searches for remote control helicpoters, drones, gliders and anything else including dirigibles/balloons, bees/moths etc
In terms of a burrowing or digging robot, you can check Youtube and the Irobot company has been selling a house eavestrough robot for about 10 years. It is about a foot long and when placed in a narrow eavestrough you use the remote control on the ground to have it move forward and back while the front spins a brush used to knock out the leaves onto the ground. You could modify something like this to go underground and possibly navigate the rough terrain.
Of course you would hate to have the underground robot get stuck 20 feet throuh a hole about a foot underground without a tracking device/GPS/magnet so you could dig straight down to retreive it.
Now ROS is the robot operating software that is being used in many or most of the worlds top robot builders.
There have been very few drone makers using ROS so far but Gaitech was one of the first