On April 9, 1983, we discovered a new explosion crater buried in the thick rain forest that covers the flanks of Arenal volcano in Costa Rica.
The previously undetected crater, which we named Crater I, is located between the twin volcanic system Arenal-Chato at 84°41′53″W and 10°27′42″N (Figure 1). It resulted from a phreatic explosion occurring probably during the 1968 explosive phase, in which three other well-known main craters (A, B, and C) were opened along a fracture on the western side of the Arenal Volcano. Crater I has formed at the physical discontinuity that existed at the boundary of Arenal's lava-armor that was created during the A.D. 1500 eruption cycle. The explosion, a small directed blast oriented 115° from the north, originated at a depth of approximately 20 m leaving a circular crater 25 m wide and 3 m deep with an axis sloping 15° from the vertical. The crater is made up of nonjuvenile lava blocks and lapilli and is now covered by thick ferns.