A particle fountain is an extremely dangerous spatial anomaly which can destroy any starship to enter it. Several Federation starships were dispatched to study one in the Alpha Quadrant, all were destroyed. Its nature is still to be determined but Starfleet are understandably reluctant to find out.
One such fountain was discovered by the USS Voyager in 2376 in the Delta Quadrant.
Note: The one discovered by the USS Voyager was found by investigation to be the home of the sentient shuttle Alice. She described it as her home but as to whether it created her or the fountain was a gateway to her home remains to be determined. Alice was destroyed when she failed to take an organic being with her into the fountain.