Chiapas  is full of caves and sinkholes (known as simas), but none compare to the gargantuan Sima de las Cotorras (Centro Ecoturístico Tzamanguimo, tel. 968/689-0289 or 968/105-6480, 7 a.m.–7 p.m. daily, US$2), about a 90-minute drive west of Tuxtla .
Almost perfectly circular, the sinkhole is 160 meters across and a stomach-lurching 140 meters straight down—like a giant posthole cut into the limestone crust. At the bottom is a small forest, a naturally protected summer nesting area for the sinkhole’s namesake residents: thousands of cotorras, or green parrots.
In the morning and evening, the birds enter and leave the sinkhole in an awesome cacophonous hurricane, flying in broad wheeling spirals and squawking incessantly. It’s a marvel to witness, and staying the night lets you see both the evening and morning shows.
While Sima de las Cotorras is impressive any time, the best time to visit is March to October, when the parrots are most numerous. (They spend the rest of the year in the warm lowlands nearer the coast.) In season, the parrots typically leave the sima at 5:30–6 a.m. and return at 4–5 p.m.
Nearly as impressive are a set of pinturas rupestres (ancient paintings) on a narrow ledge right in the middle of the sheer cliff wall. The ledge circles around the sima, climbing slowly to the lip; to see the paintings up close, it’s a knee-knocking 200-meter walk down the ledge, with 70 meters of sheer cliff above and below you.
Local guides will take you there for about US$5; better yet, on Saturdays and (most) Sundays, a rock-climbing club guides visitors to the paintings with the assurance of a fixed safety rope set up for the purpose (US$6). The same team offers rappelling (US$30 to the ledge, US$60 to the sima floor). The climb back up is extremely strenous.
There’s a second and smaller sima a half-hour walk from the main one, also with ancient paintings in it. If you simply want to admire the view, there’s a trail leading around Sima de las Cotorras that takes about 20 minutes to complete, with a number of good vista points along the way.
A cluster of new and very pleasant cabins (US$25–55 for 2–4 people) are set a short distance into the trees; each includes one to two bedrooms and large hot-water bathrooms. Sturdy stone exteriors give way to terra-cotta floors, wood beams, and even patios with hammocks.
The restaurant (7 a.m.–8 a.m. daily, US$3–7) is built right on the edge of the sinkhole, with excellent views and decent food.
Driving from Tuxtla  on Highway 190, look for signs for Sima de las Cotorras as you pass through the small town of Ocozocoautla. From the turnoff, it’s 3.5 kilometers on a paved road to Crucero San Luis, where a huge sign directs you left onto a dirt road. It’s another 12 kilometers from there, passing through the small town of Piedra Parada (home to a huge family restaurant called El Borrego Líder). The road is in decent shape, though it is muddy and slippery after heavy rains.
You can theoretically catch a round-trip shuttle (US$10 pp, children 2-for-1, escobar250_9 [at] hotmail [dot] com) from Parque Marimba in Tuxtla  on Saturday and Sunday at 11:50 a.m. (and daily during vacation periods). The trip lasts six hours, including a stop at Ocuilapa, a Zoque community known for its pottery. Ten people are required for the shuttle to depart, so low-season departures may be unreliable; ask at the tourist office in Parque Marimba for the latest.