Sotano de las Golondrinas is not in Xilitla proper, but many travellers visit it from town, and it is one of the Huasteca's great natural spectacles. The open pit drops hundreds of metres, and birds pour out of it in a widening spiral at dawn before returning near dusk.
The timing matters. Arrive very early or very late, and expect a rougher, more rural excursion than a standard viewpoint stop.