Huge land of stairs

Please force a crash by pressing and holding F3+C for 10 seconds.

---- Minecraft Crash Report ----
// Surprise! Haha. Well, this is awkward.

Time: 11/20/13 7:21 PM
Description: Manually triggered debug crash

java.lang.Throwable
	at azd.o(SourceFile:1383)
	at azd.ad(SourceFile:753)
	at azd.e(SourceFile:704)
	at net.minecraft.client.main.Main.main(SourceFile:103)


A detailed walkthrough of the error, its code path and all known details is as follows:
---------------------------------------------------------------------------------------

-- Head --
Stacktrace:
	at biz.a(SourceFile:284)

-- Affected level --
Details:
	Level name: MpServer
	All players: 1 total; [bje['flash_bomber'/274, l='MpServer', x=-1004.12, y=70.62, z=643.52]]
	Chunk stats: MultiplayerChunkCache: 287, 287
	Level seed: 0
	Level generator: ID 00 - default, ver 1. Features enabled: false
	Level generator options: 
	Level spawn location: World: (-240,64,248), Chunk: (at 0,4,8 in -15,15; contains blocks -240,0,240 to -225,255,255), Region: (-1,0; contains chunks -32,0 to -1,31, blocks -512,0,0 to -1,255,511)
	Level time: 40330 game time, 3809 day time
	Level dimension: 0
	Level storage version: 0x00000 - Unknown?
	Level weather: Rain time: 0 (now: false), thunder time: 0 (now: false)
	Level game mode: Game mode: survival (ID 0). Hardcore: false. Cheats: false
	Forced entities: 98 total; [vw['item.tile.stone'/275, l='MpServer', x=-988.03, y=16.13, z=665.16], vw['item.tile.stone'/277, l='MpServer', x=-988.91, y=19.13, z=661.16], vw['item.tile.stone'/276, l='MpServer', x=-988.19, y=19.13, z=658.31], wx['Skeleton'/2198, l='MpServer', x=-974.94, y=17.00, z=590.56], uq['Bat'/1659, l='MpServer', x=-963.22, y=42.00, z=698.03], xc['Zombie'/2959, l='MpServer', x=-972.50, y=27.00, z=573.50], wl['Creeper'/1399, l='MpServer', x=-987.13, y=15.00, z=636.75], xc['Zombie'/2968, l='MpServer', x=-986.50, y=63.00, z=672.50], wx['Skeleton'/2963, l='MpServer', x=-971.50, y=29.00, z=613.50], xc['Zombie'/340, l='MpServer', x=-1061.50, y=18.00, z=676.50], vw['item.tile.dirt.default'/337, l='MpServer', x=-1059.78, y=62.28, z=602.88], ve['Squid'/336, l='MpServer', x=-1064.28, y=55.34, z=582.16], vw['item.tile.torch'/351, l='MpServer', x=-1052.66, y=38.13, z=607.31], wx['Skeleton'/349, l='MpServer', x=-1041.50, y=45.00, z=572.50], vy['entity.MinecartChest.name'/348, l='MpServer', x=-1054.50, y=39.50, z=567.50], vy['entity.MinecartChest.name'/324, l='MpServer', x=-1081.50, y=38.50, z=605.50], vb['Sheep'/322, l='MpServer', x=-1080.50, y=66.00, z=585.66], uq['Bat'/635, l='MpServer', x=-963.63, y=32.13, z=718.47], ve['Squid'/335, l='MpServer', x=-1063.34, y=58.39, z=583.09], uq['Bat'/1590, l='MpServer', x=-1040.91, y=47.51, z=643.06], vw['item.item.seeds'/380, l='MpServer', x=-1016.53, y=67.13, z=583.19], uq['Bat'/383, l='MpServer', x=-1014.75, y=21.10, z=655.25], wx['Skeleton'/1542, l='MpServer', x=-1045.50, y=15.00, z=699.50], vy['entity.MinecartChest.name'/382, l='MpServer', x=-1011.50, y=19.50, z=650.50], vw['item.tile.rail'/356, l='MpServer', x=-1037.13, y=46.13, z=598.97], ve['Squid'/357, l='MpServer', x=-1035.50, y=51.22, z=602.50], ve['Squid'/358, l='MpServer', x=-1040.22, y=52.34, z=596.50], vw['item.tile.rail'/352, l='MpServer', x=-1048.09, y=38.13, z=608.25], vw['item.tile.stone'/353, l='MpServer', x=-1054.88, y=34.13, z=662.81], wl['Creeper'/365, l='MpServer', x=-1035.47, y=51.00, z=640.34], xc['Zombie'/366, l='MpServer', x=-1039.50, y=31.00, z=667.50], wl['Creeper'/367, l='MpServer', x=-1039.50, y=63.00, z=703.50], wl['Creeper'/1315, l='MpServer', x=-940.50, y=8.00, z=674.50], wl['Creeper'/360, l='MpServer', x=-1030.50, y=18.00, z=636.50], wl['Creeper'/1316, l='MpServer', x=-939.50, y=8.00, z=677.50], uq['Bat'/363, l='MpServer', x=-1039.97, y=43.41, z=639.43], vw['item.tile.rail'/410, l='MpServer', x=-983.94, y=11.13, z=615.28], vw['item.tile.rail'/411, l='MpServer', x=-976.19, y=14.13, z=627.88], uq['Bat'/414, l='MpServer', x=-984.47, y=13.10, z=652.25], wl['Creeper'/415, l='MpServer', x=-995.47, y=19.00, z=649.63], vw['item.item.string'/412, l='MpServer', x=-976.50, y=14.13, z=627.19], wl['Creeper'/413, l='MpServer', x=-985.03, y=15.00, z=646.38], vw['item.tile.rail'/402, l='MpServer', x=-994.13, y=19.13, z=671.16], vw['item.tile.rail'/403, l='MpServer', x=-995.75, y=19.13, z=673.88], vw['item.tile.rail'/400, l='MpServer', x=-996.88, y=19.13, z=653.13], vw['item.tile.rail'/401, l='MpServer', x=-994.13, y=19.13, z=663.88], vw['item.tile.rail'/404, l='MpServer', x=-996.88, y=19.13, z=673.88], wx['Skeleton'/405, l='MpServer', x=-1007.50, y=27.00, z=711.50], wx['Skeleton'/1236, l='MpServer', x=-1018.69, y=40.00, z=577.59], wl['Creeper'/1478, l='MpServer', x=-1002.50, y=33.00, z=721.50], vw['item.tile.rail'/399, l='MpServer', x=-994.13, y=19.13, z=653.13], xc['Zombie'/1233, l='MpServer', x=-1016.50, y=40.00, z=574.50], vy['entity.MinecartChest.name'/398, l='MpServer', x=-1001.50, y=19.50, z=640.50], vw['item.tile.rail'/397, l='MpServer', x=-994.13, y=15.13, z=636.78], xc['Zombie'/1234, l='MpServer', x=-1018.50, y=40.00, z=576.50], xc['Zombie'/1235, l='MpServer', x=-1019.69, y=40.00, z=577.41], wx['Skeleton'/396, l='MpServer', x=-995.06, y=13.00, z=576.50], wx['Skeleton'/2331, l='MpServer', x=-965.50, y=53.00, z=590.09], xc['Zombie'/386, l='MpServer', x=-1023.59, y=56.00, z=663.00], xc['Zombie'/385, l='MpServer', x=-1014.00, y=30.00, z=659.41], wl['Creeper'/2334, l='MpServer', x=-968.50, y=54.00, z=582.50], wl['Creeper'/2332, l='MpServer', x=-969.50, y=54.00, z=584.50], xc['Zombie'/433, l='MpServer', x=-964.50, y=14.00, z=626.50], vw['item.tile.rail'/434, l='MpServer', x=-966.88, y=14.13, z=653.88], vw['item.tile.rail'/435, l='MpServer', x=-966.16, y=14.13, z=651.41], vw['item.tile.rail'/436, l='MpServer', x=-964.13, y=14.13, z=646.81], vw['item.tile.rail'/437, l='MpServer', x=-966.88, y=14.13, z=646.81], xc['Zombie'/438, l='MpServer', x=-971.59, y=17.00, z=649.91], vw['item.tile.rail'/439, l='MpServer', x=-974.59, y=15.13, z=661.00], uq['Bat'/425, l='MpServer', x=-966.83, y=52.04, z=590.56], vw['item.tile.torch'/426, l='MpServer', x=-975.13, y=14.13, z=617.81], wz['Spider'/428, l='MpServer', x=-964.03, y=20.99, z=612.28], xc['Zombie'/2568, l='MpServer', x=-1041.50, y=31.00, z=664.50], va['Pig'/421, l='MpServer', x=-970.50, y=102.00, z=565.47], xc['Zombie'/2569, l='MpServer', x=-1039.50, y=31.00, z=669.50], va['Pig'/420, l='MpServer', x=-965.31, y=79.00, z=571.50], va['Pig'/422, l='MpServer', x=-970.09, y=103.00, z=566.06], wl['Creeper'/1974, l='MpServer', x=-1051.50, y=66.00, z=661.50], xc['Zombie'/2924, l='MpServer', x=-1041.50, y=43.00, z=633.50], wl['Creeper'/467, l='MpServer', x=-940.50, y=47.36, z=583.97], xc['Zombie'/458, l='MpServer', x=-953.50, y=55.00, z=716.50], xc['Zombie'/2114, l='MpServer', x=-959.50, y=30.00, z=599.50], wx['Skeleton'/457, l='MpServer', x=-949.13, y=41.00, z=674.50], xc['Zombie'/2115, l='MpServer', x=-957.50, y=30.00, z=601.50], wx['Skeleton'/456, l='MpServer', x=-945.88, y=41.00, z=679.50], uq['Bat'/455, l='MpServer', x=-940.25, y=18.71, z=622.75], vw['item.tile.stone'/454, l='MpServer', x=-947.88, y=20.13, z=615.91], vy['entity.MinecartChest.name'/453, l='MpServer', x=-951.50, y=18.50, z=617.50], uq['Bat'/450, l='MpServer', x=-957.25, y=27.10, z=603.31], uq['Bat'/2165, l='MpServer', x=-935.50, y=31.48, z=587.47], bje['flash_bomber'/274, l='MpServer', x=-1004.12, y=70.62, z=643.52], wz['Spider'/1458, l='MpServer', x=-1017.50, y=41.00, z=596.50], wx['Skeleton'/1464, l='MpServer', x=-932.50, y=19.00, z=627.50], wx['Skeleton'/1465, l='MpServer', x=-929.50, y=19.00, z=627.50], xc['Zombie'/1680, l='MpServer', x=-942.09, y=17.00, z=638.41], xc['Zombie'/1681, l='MpServer', x=-948.50, y=16.00, z=634.06], xc['Zombie'/1682, l='MpServer', x=-942.50, y=17.00, z=629.50], wl['Creeper'/2639, l='MpServer', x=-939.50, y=75.00, z=666.50]]
	Retry entities: 0 total; []
	Server brand: vanilla
	Server type: Integrated singleplayer server
Stacktrace:
	at biz.a(SourceFile:284)
	at azd.b(SourceFile:1951)
	at azd.e(SourceFile:713)
	at net.minecraft.client.main.Main.main(SourceFile:103)

-- System Details --
Details:
	Minecraft Version: 1.7.2
	Operating System: Windows 7 (amd64) version 6.1
	Java Version: 1.7.0_45, Oracle Corporation
	Java VM Version: Java HotSpot(TM) 64-Bit Server VM (mixed mode), Oracle Corporation
	Memory: 190777136 bytes (181 MB) / 434634752 bytes (414 MB) up to 3817865216 bytes (3641 MB)
	JVM Flags: 2 total; -XX:HeapDumpPath=MojangTricksIntelDriversForPerformance_javaw.exe_minecraft.exe.heapdump -Xmx4096M
	AABB Pool Size: 24227 (1356712 bytes; 1 MB) allocated, 2 (112 bytes; 0 MB) used
	IntCache: cache: 0, tcache: 52, allocated: 12, tallocated: 94
	Launched Version: 1.7.2
	LWJGL: 2.9.0
	OpenGL: Quadro FX 1700/PCIe/SSE2 GL version 3.3.0, NVIDIA Corporation
	Is Modded: Probably not. Jar signature remains and client brand is untouched.
	Type: Client (map_client.txt)
	Resource Packs: []
	Current Language: English (US)
	Profiler Position: N/A (disabled)
	Vec3 Pool Size: 3517 (196952 bytes; 0 MB) allocated, 55 (3080 bytes; 0 MB) used
	Anisotropic Filtering: Off (1)

PS: Even though it says MPServer i'm on singleplayer.

Someone has replaced dirt by jungle wood stairs.
Please zip the world and attach it to this ticket.

PS: Even though it says MPServer i'm on singleplayer.

Indeed, Minecraft runs an internal server (which allows for LAN, among other things).

What's important really is whether or not your game was modded, which it is not so that's ok.