The Borneo rainforest is 130 million years old, making it one of the oldest rainforests in the world and 70 million years older than the Amazon rainforest. Borneo is very rich in biodiversity compared to many other areas. There are about 15,000 species of flowering plants with 3,000 species of trees, 221 species of mammals and 420 species of birds. Subject to mass deforestation, the remaining Borneo rainforest is one of the only remaining natural habitats for the endangered Bornean orangutan. It is also an important refuge for many endemic forest species, as the Asian elephant, the Sumatran rhinoceros, the Bornean clouded leopard, and the Dayak fruit bat.

Kalimantan has an equatorial climate, with the average temperature ranging between 23 and 31°C. The island receives around 300mm of rain per year, with a light rainy season between March and May, and a more intense one between November and January. Rainfall however considerably lower than in many neighbouring parts of Indonesia.

There are several international airports in Kalimantan, with connections to and from Malaysia. In addition, there is a wide array of flight connections to and from other Indonesian islands, mainly Java.

• Supadio Airport (PNK ^IATA) near Pontianak is the primary airport on the Indonesian side, with international flights from Kuala Lumpur (AirAsia) and domestic flights from Medan and the main cities of Java.
• Syamsudin Noor Airport (BDJ ^IATA) near Banjarmasin is an airport located in South Kalimantan, with good connections to destinations throughout Kalimantan and Java.
• Sultan Aji Muhammad Sulaiman Sepinggan Airport (BPN ^IATA) near Balikpapan with domestic flights from many Indonesian cities.
• Samarinda APT Pranoto International Airport (AAP ^IATA)
• Juwata Airport (TRK ^IATA) near Tarakan servers the northeast of Kalimantan, with a range of domestic destinations.
• Also the airports of Tanjung Redeb (BEJ ^IATA), Pangkalan Bun (PKN ^IATA), Ketapang (KTG ^IATA), Sampit (SMQ ^IATA), Palangkaraya (PKY ^IATA) serve destinations on Java island.

The only formal way to enter Kalimantan by land from Malaysia is at the Entikong-Tebedu crossing between West Kalimantan and Sarawak. As the crossing is listed only as a visa-free entry point, nationalities who do not qualify for this will have to apply for visas beforehand. Read more about this in the article about the trip from Pontianak to Kuching or vice versa.

There is an international daily ferry connection from Tawau in Sabah, Malaysia, to Nunukan and Tarakan in North Kalimantan province.

Domestic ferry connections to several ports of Kalimantan (Pontianak, Ketapang, Kumai, Sampit, Banjarmasin, Batulicin, Balikpapan, Samarinda, Tarakan, and Nunukan) are operated by PELNI. In almost all cases it is advisable for tourists to fly instead, except perhaps for trips to Kalimantan from relatively nearby islands such as Bangka-Belitung and the Riau Islands.

Most diving activities takes place in the Berau Archipelago in the Celebes sea, around the islands of Derawan, Sangaliki and Maratua in East Kalimantan, there are also some less visited dive sites in West Kalimantan, these include Batu Payung and Radayan island in the vicinity of Singkawang and Karimata islands reserve further south, some 100 kilometers of the coast from Ketapang.