Preliminary Note on the Floating Islands
of Zacaton Sinkhole, Mexico

by Chet Van Duzer, 12177 Winton Way, Los Altos Hills, California 94024; E-mail: ChetV@aol.com

On El Rancho Azufrosa near the small town of Aldama (22^o 55'N, 98^o 04'W) in the state of Tamaulipas in northeastern Mexico, there is a remarkable group of five cenotes or sinkholes, vertical caves filled with fresh water. The water in the sinkholes is highly mineralized, smelling strongly of sulfur, and is also quite warm, with average temperatures ranging from 28.3^oC to 33.8^oC. One of these cenotes, called Zacaton, is the world's deepest known water-filled pit, more than 305 meters deep, and is the site of the world's deepest scuba dive, which was made by Jim Bowden, leader of El Proyecto de Buceo Espeleologico Mexico y America Central, a group of divers which has been exploring the sinkholes since 1989.

While the depths of Zacaton are of speleological interest, its surface is of botanical interest for the lush floating islands that move across it. The cenote's surface is circular, about 100 m in diameter, and is surrounded by 21 m high rocky cliffs. On the water are fifteen floating islands, ranging in diameter from 3 to 10 m, and 1 to 1.5 m thick. Beneath the water, the edges of the islands are essentially vertical, a result of the islands' collisions with each other and with the vertical rock "shores." The islands are moved only by the wind; there are no currents in Zacaton.

The flora of the floating islands is dominated by a grass known as "zacate," and in fact it was the distinctive islands of zacate that gave the cenote its name "Zacaton. "This grass has not yet been collected and identified. The names "zacate" and "zacaton" are applied to several different species, including Muhlenbergia robusta, Festuca amplissima, and Sporobolus wrightii, as well as other species in these genera. A Sporobolus grass seems the most likely candidate, as Muhlenbergia spp. and Festuca spp. are typically found in dry environments, while Sporobolus spp. are known to grow in desert marshes, playa lakes, and floodplains. A small number of shrubs and cacti also grow on the islands, and the islands are inhabited by turtles and snakes. I have heard a report of floating islands of zacate grass which are called "zacatones" in Laguna Verde near Coapilla (93^o9'59"W, 17^o7'59"N), Chiapas, Mexico. These islands might prove interesting to compare with those of Zacaton, but information about the islands in Laguna Verde has not been forthcoming.

Perhaps the most interesting question raised by the floating islands of Zacaton is how they formed. There are no shelves near the water's surface on which a colony of grass might grow, become dislodged, and float, and indeed there are no stands of this species= of grass in the immediate vicinity of the sinkhole. Further, there are no shallow underwater shelves upon which humus might have accumulated, become buoyant due to decompositional gasses, and then been colonized by the grass. Marcus Gary, a hydrologist with the U.S. Geological Survey who is studying the Rancho Azufrosa sinkholes, has suggested to me that the islands may have formed on buoyant "skins" of travertine, a precipitate of calcium carbonate. There are other sinkholes in the area that are now filled with travertine deposits. It seems that the chemistry of Zacaton's waters has changed so that travertine is no longer forming, and may in fact be dissolving, but the islands remain. Over time, dust would have accumulated on these travertine rafts, and the grass seeds might have been carried to Zacaton by birds -- this area is well known among birdwatchers, and many different species of birds live in and around the cenotes.

Other floating islands have formed on travertine rafts. A lake now called Lago della Regina, and formerly known as Lacus Albuleus, La Solfatra, or Lago delle Isole Natanti, near Tivoli, Italy, once had vegetated floating islands formed on floating masses of travertine. These were famously described by Athanasius Kircher and Francesco Lana in the 17th century, and in more detail by Sir Humphry Davy in the 19th century, not long before they ceased to exist, probably because water was diverted from the lake to supply thermal baths. Lana describes these floating islands as follows: "I myself saw several of these islands in a small lake of sulfurous water not far from the Tiber; they were mostly circular or oval, and rose four or six inches above the water. Their surface is flat and grassy, and at the edges of some of them a few larger plants grow, which act as sails, so that even the slightest breeze pushes the islands from one part of the lake to another. The largest of them are a few yards in diameter, yet nonetheless can sustain several men standing upon them."

Hopefully an opportunity for a thorough investigation of the floating islands of Zacaton, including a survey of their flora and fauna, will present itself soon.

Brand, Charles J., and Merrill, Jason L., "Zacaton as a Paper-making Material," United States Department of Agriculture Bulletin No. 309 (November 4, 1915) (28 pp.).


Brown, David E., "Chihuahuan Desertscrub," p. 169-79 in David E. Brown, ed., Biotic Communities of the American Southwest -- United States and Mexico (Tucson, 1982) (p. 175 on the occurrence of Sporobolus wrightii and Sporobolus airoides in wetland communities).


Davy, Humphry, Sir, Consolations in Travel, or, The Last Days of a Philosopher (London, 1830) (p. 122-9 gives an account of the floating islands in La Solfatara or Lago della Regina).


Gary, Marcus, "Speleogenesis of Zacaton and Cenotes of Rancho La Azufrosa," Poster Presentation, American Academy of Underwater Sciences 20th Annual Symposium, "Diving for Science in the 21st Century," 11 to 15 October 2000, Sirata Beach Resort, St. Petersburg Beach, Florida.


Kircher, Athanasius, Latium; id est, Nova & parallela Latii tum veteris tum novi descriptio (Amsterdam, 1671) (Book 4, Part 3, chapter 4 on the floating islands in the Lago della Regina).


Kristovich, Ann, "Zacaton. A History," Nitrox Diver Magazine 94.4 (Nov. 1994 - Jan. 1995) (on scuba diving in Zacaton; online at http://www.iantd .com/articles/94-4kristovich.html")


Lana Terzi, Francesco, Magisterium naturae, et artis (Brescia, 1684-92) (Vol. 3, Book 25, chapter 1, number 54 on the floating islands in the Lago della Regina).

Bacci, Andrea, Discorso delle acque Albule, bagni di Cesare Augusto a' Tivoli, delle acque acetose presso a Roma, & delle acque d'Anticoli (Rome, 1564) (32 p.; esp. p. 3-4 on the qualities of the water and on the floating islands in Lago della Regina).


Cappello, Agostino, De' bagni minerali presso Tivoli (Rome: Tipofrafia delle belle arti, 1839) (29 p., offprint from Giornale Arcadico, vol. 80; p. 15-7 on the floating islands).


Kircher, Athanasius, Latium; id est, Nova & parallela Latii tum veteris tum novi descriptio (Amsterdam, 1671) (describes the floating islands near Tivoli in Book 4, part 3. chapt. 4.; the islands were known as le sedici barchette, "the sixteen little boats," and are mistakenly depicted as boats on the map of the lake and surrounding regions in Book 3, part 2, chapt. 1).


Gigli, Girolamo, Il Gorgoleo ovvero il governatore dell'isole natanti (Sienna, 1753) (a comedy; in Act 1, Scene 2, p. 14-5 the characters discuss the floating islands of Acque Albule near Tivoli, mentioning some of the names of the individual islands, and that local shepherds ride on the islands).


Viale, Benedetto, and Latini, Vincenzo, Sulle Acque Albule presso Tivoli: Analisi chimica (Rome: Tipografia di Gaetano Menicanti, 1857) (76 p.; p. 12-4, 49, 69, and 74 on the floating islands).


Zezi, Pietro, "The Travertine and the Acque Albule in the= Neighbourhood of Tivoli," p. 83-8 in Henry James Johnston-Lavis, ed., The South Italian Volcanoes, Being the Account of an Excursion to Them Made by English and Other Geologists in 1889 Under the Auspices of the Geologists' Association of London (Naples, 1891) (p. 85-6 on the Lago della Regina, with brief reference to its floating islands).

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