How can make a hole in the ground to connect above-ground map to below?

So I have an above-ground map in a level that doesn’t contain anything “underneath” or underground. I purchased an underground map and want to combine the two maps into one same seamless level to allow for walking between the above and below ground without cut-scenes, loading screens, etc… and i’ve build an escalator system to bridge the two, however the above ground map has ground collision, and the ground is one single large object with textures and grass everywhere…

How can I basically punch a hole intm the ground to 1) get rid of the ground texture and grass where the hole should be and also 2) get rid of the collison detecting there so that I can use that gaping hole to connect a direct escalator from above ground map to my underground map all within the one sam level?

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