A group of six Monk seals (Monachus monachus) was discovered to lie up inside a grotto on the shores of a lake reached via an underwater passage. Nearly 1200 hours of observation on the movement of the seals showed that the greatest use of the grotto was made during the winter months and when the wind was towards the grotto entrance. Entry and exit times were not significantly affected by sea state, dark or light, nor were they significantly correlated with sunrise and sunset times or sea and air temperature. These variables did not significantly affect the length of stay in the grotto. Entry occurred most frequently in the early morning and exist at night. Discussion is given of the possible reasons why grottos are used by Monk seals. This appears to be a recent adaptation which may be related to the effects of human disturbance.