The workers were all big, burly, hard-hearted men, tromping through the marsh in their heavy boots without sparing so much as a single thought for the masses of tiny frogs they crushed underfoot.
under the feet; "trampled the beans underfoot"; "green grass growing underfoot" in the way and hindering progress; "a house with children and pets and toys always underfoot
You describe something as being underfoot when you are standing or walking on it. a room, high and square with carpet underfoot and tapestries on the walls It was still wet underfoot