How can you perceive burrowing?

Question

So, I ran across a video saying that some burrowing creatures can be used as mounts by ...either gnomes or forest elves, I forget which, in 5e. Well, sure it will be slow, but according to what I know, it's essentially a bypass for all wall standard walls, and while underground, a free stealth check, unless the enemy has tremorsense (which is rare in and of itself, and not often very far-ranging).

My question to you guys is: How could you perceive, or would you even have a chance to perceive, a burrowing enemy under these 2 conditions: when you're expecting them, and when you're not.

Answer 1

Your options for this are surprisingly limited. To perceive a burrowing creature yourself, you can:

• Elemental Wild Shape into an Earth Elemental.
• Shapechange into an Ankheg, Bulette, Earth Elemental, Galeb Duhr, Purple Worm, Remorhaz, Umber Hulk, or Xorn.

To have something perceive it for you:

• Use Conjure Elemental to get an Earth Elemental, Galeb Duhr, or Xorn.
• Use Dominate Monster on an Ankheg, Bulette, Earth Elemental, Galeb Duhr, Purple Worm, Remorhaz, Umber Hulk, or Xorn.

To find out where something is without actually perceiving it:

• If you're expecting something, you can use Alarm to detect it when it gets close (although this won't give you much warning).
• A Mordenkainen's Faithful Hound might work the same way.
• If you're expecting something, you're on dirt, not rock, and it is a thinking creature, you can use Detect Thoughts to find out where it is and what it is thinking (although this only works within 30 feet).
• If you're expecting something and you know that it will have a particular object on its person, you can use Locate Object to know its direction from you and the direction of its movement.
• If you're expecting something and you know what kind of creature it is, you can use Locate Animals or Plants to know its direction and distance from you.
• If you're expecting something and you know what kind of creature it is, you can use Locate Creature to know its direction from you and the direction of its movement.
• You can ask any question, including questions about burrowing creatures, using Divination, Commune, or Contact Other Plane.

As you can see, it's quite hard to see burrowing enemies coming. However, the reverse is equally true: it would be quite difficult for a burrowing assassin to know where you are. That simplifies things a long way, because as long as you keep moving, the halfling assassin riding a giant badger (or whatever) won't know where you are, either. If you want to stand still, it's probably a good idea to use Alarm.

Answer 2

This is a situation where the "living rules" of 5e really kick in; there are no express rulings I know of considering the detection of burrowing animals, so you'll have to house-rule this. (Really, I checked their twitters.) Here are some things which you ought to consider.

1. The earth transmits sound rather well. This is why you can hear large groups of things or heavy things moving by putting your ear to the ground. This is true even for very large distances.
2. The earth transmits vibrations decently well. If a burrowing creature, especially one that is larger than your normal mole, will shake the earth enough that people will eventually notice, and usually before the animal shows up.

Given that this is movement-concerning, I feel a perception check would be ideal, with increasing difficulty the further away the burrowing creature is. Give advantage to those who have their ears to the ground. This works for both of those situations.