The Window Cave is located on HW10 about the same distance from Utuado and Arecibo. The entrance is actually not hard to spot from the road and it is located next to a gas station, with a bathroom to which seems like no one had a key.
After paying $5 per person and having a short 10 minutes hike you will arrive at the entrance of the cave. There actually two caves there, side by side, one is a huge almost like a dome and the second one is the Window Cave. This photo shows the first chamber of the Window Cave, it leads you through a pitch dark halls to the huge opening, showing the valley bellow. The pitch dark halls and chambers are full with bats, they are hanging from the ceiling in huge batches and fly all around. Pretty cool place to visit.
I used my headlamp to light up this cave, had to move it in circular motion during long exposures, to get it evenly.
And yep I did take couple photos of the window too, just have not had a chance to process it yet.