If there is a *known* ground below, the stone can only pumlI' .
Of course if you don't know how deep it is, and can't see the ground, then pumtaH would be fine - from your point of view the stone might just as well fall forever.
But if we can see the ground and you say "pumtaH" to me, I'd expect the stone to fall through the earth and continue to fall until something intentionally blocks its way.

TKD tells us that using -lI' instead of -taH is optional (whenever the ideas behind -lI' apply). You can always use -taH for any kind of ongoing action. So if I can say De' vIlI'lI' I am transmitting data, I can also say De' vIlI'taH.

So pumtaH nagh doesn't mean the stone won't stop when it hits the ground; it just ignores whatever the stopping point may be.

-- 
SuStel
http://trimboli.name