The Gritzy Caves is a giant cave right under the Koopaseum in Mario & Luigi: Partners in Time. This was once Petey Piranha's home. Monty Moles run a Item Shop here too.
After Shroobs hit Mario, Luigi, Baby Mario and Baby Luigi they all fell down to the Gritzy Caves. Then, they found Kylie Koopa, who got eaten by Petey Piranha.
Mario and Company later fought Petey Piranha. Kylie, a Cobalt Star Shard and Peach flew out of Petey Piranha.