During the Eocene, the Indian subcontinent was an island just beginning its collision with Asia which would eventually lead to the uprising of the Himalayas. The Eocene had a greenhouse climate (no permanent ice sheets at the poles) as opposed to the icehouse climate of today, so, in general, areas were much warmer than today. The abundance of Eocene brown coal deposits preserving tropical biota on the Indian subcontinent indicates the proliferation of tropical rainforests in a hot climate. Mangroves seem to have commonly grown along the western margin of the subcontinent in the early Eocene, but decreased nearing the Middle Eocene Climatic Optimum. The waters off the western coast seem to have featured upwelling and low-oxygen.