In Peñablanca, the Callao Cave Cathedral is well known and the most-visited natural attraction of Cagayan. Easily accessible by car, Callao Cave has 7 chambers to explore. From the carpark, take a few steps and start to ascend at the "catch-your-breath" 183 steps (as per my son, Baggi's count) on a cemented stair; and Bingo. Offer a prayer at the chapel located at the 1st chamber, the biggest with its natural skylight. Then explore the next chambers into the cave. Awesome stalactites/stalagmites formation. Spot the "eagle's head with the beak", "lion's head", "missile rocket" - just a few of the formations you can imagine.
When exploring Callao Cave, it should be noted that caves are generally wet inside due to the continuous dripping of water from the mountain making its way through the porous rocks making the cave's ground slippery muddy - choose the right footwear. Flashlights will also be of big help. Within the vicinity, a lot more of caves can be explored by the more adventurous skilled ones (Bring your gear).